Starter items

Core items

Offensive items

Defensive items

Situational items

I am Mystery, just a normal player on Infinite Crisis, and I wanted to create a guide for my favourite champion, Harley Quinn.
When I first started playing her I realised that she had lots of potential, I loved her skill set and her play style. Give her a go, she is fun to play :D.

PREPARE FOR LOTS OF TEXT AND STUFF, BRACE YOURSELF!!!
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How do i play Harley?
I play Harley as a support/roaming champion (usually at the bot lane), her ganks and initiations are amazing if you manage to hit all of her abilities right. Other than that I will always make sure to keep the ADC safe, especially late game. Keeping the ADC alive during a team fight is crucial, and Harley does an AMAZING job at keeping teammates alive and helping them escape bad situations. As long as I manage to keep the ADC alive, most team fights are won.
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Good with

Atomic Joker

Cyborg

Gaslight Batman

Green Arrow

Mecha Wonder Woman

Catwoman

Joker

Harley works well with any ADC champions, she also works well with Poke champions. ADC's rely on Harley for safety and protecting allies is one of Harley's main goals. Poke champions like the Joker and Catwoman offer great damage from a far distance but they generally lack defence, and this is where Harley comes in as she can defend these champions from ganks and initiations whilst turning fights around. If someone picks one of these champions then Harley Quinn is a very good choice to go with them.
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Bad against

Atomic Wonder Woman

Batman

Flash

Gaslight Catwoman

Nightmare Robin

Mecha Superman

Nightmare Batman

Harley is squishy and even with defensive items she can be killed easily, especially by assassins. Harley is a big target as she has some of the best CC skills in the game. If you are Harley and someone picks one of these then make sure you all co-operate together as a team. If your team does not defend you, then you are going to have a bad time defending them. She is also bad against gap closing champions as she is not a very good escaper on her own, any champions with gap closers and CC can lock Harley down allowing her to be targeted by the entire enemy team.
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Pros
Amazing crowd control abilities which can set your team up for kills.
Amazing at roaming and ganking lanes, she can CC literally a whole lane to allow for easy kills.
Scales well with Will (which also comes in defensive items).
Great at clearing jungle monsters mid-late game with pies.
Great movement speed steroid with her passive BFFS, helps not only you but allies escape bad situations as well.
Great mid-late game.

Cons
Squishy until you build some defensive items.
Her only damaging abilities are really Q and R, low sustained damage.
Almost worthless without backup from teammates.
High cooldown abilities until you get cooldown reduction.
Often focused (which is why we buy tank items).
Very team reliant, relies on a good team.
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Abilities

Pies! (Q)
Harley Quinn throws pies at the 3 targets closest to her. Each pie deals 50 Power Damage.
Cooldown: 8.0s 25 Will
Damage: 50 > 100 > 150 > 200 + 40% Power Damage + 4% Bonus Will
Cooldown: 9 > 8 > 7 > 6
Cost: 25 > 35 > 45 > 55

As you can see, this ability is Harley's main damaging ability. It has no CC so keep that in mind.
This ability scales off of WILL, which is amazing. It does not seem like much Will, but it really helps as your defensive items will have Will boosts too >:). A total of 2300 bonus Will is from this build, 4% of that is 92 bonus damage, this is very helpful as you will have also boosted your defence and Power in the process and Power boosts it even more :D.

I max this first to do the most damage possible to enemies, it does surprisingly a lot of DPS even though you are more of a support champion and does not cost much Will at all allowing you to spam it often (but don't spam it too often).
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Back off (W)
Harley Quinn Knocks Back her target, deals 60 Power Damage, and then Slows them by 25% (+1% per 100 Bonus Will) for 2s.
Cooldown: 16.0s 65 Will
Damage: 60 > 110 > 160 > 210 + 40% Power Damage
Cost: 65 > 70 > 75 > 80

This ability actually does more damage than pies by a very little amount, but not in terms of overall DPS. This ability is your main one which sets up kills. Your ultimate just stuns the enemies allowing you to get in position to use this ability. Managing to knock an enemy back into your team (preferably the ADC) is very huge and can win you a game. This ability also slows the target by 25%, and the slow increases by +1% per 100 bonus Will meaning that with a full build it will be around a 48% slow in total so use it wisely. If you hit a target into a wall they will take additional damage and will be briefly stunned, so try to angle it towards a wall if possible.

I max this last, like all crowd control abilities I use them for the crowd control, not really the damage.
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Best Medicine (E)
Cooldown: 10.0s 40 Will
Harley Quinn heals her ally for 40 Health and they receive the BFFs effect.
Heal: 40 > 70 > 100 > 130 + 10% Bonus Will
Cost: 60 > 90 > 120 > 150

I love this ability, not just because it heals but because it also boosts itself by bonus Will so with a full build you will heal 360 health every 6 seconds. This ability costs a lot of Will and you cannot afford to use all of your will with it. Only use it when necessary, an example is if your lane partner is low on health or if an ally is being chased and you can't attack anyone for the BFFS passive.

I max this second, more sustain is amazing to have in the lane because you can stay in the lane longer for more gold, experience and even kills.
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Puddin' Time (R)
Cooldown: 120.0s
Harley Quinn smashes her hammer to the ground, dealing 175 Power Damage in a cone. Enemies caught in the cone are Knocked Up for 1.5s.
Damage: 175 > 225 > 275 > 325 + 70% Power Damage
Cooldown: 120 > 110 > 100 > 90

This ability does not do much damage believe it or not, it helps when bursting down a squishy target but don't expect it to do much damage, use it for the crowd control side instead. The enemies get knocked up for 1.5 seconds in a large cone area, this does NOT increase per level, only the cooldown and the damage increases. I still rank it up anyway, a 90 second cooldown ultimate with more damage is more helpful for your team as it allows you to gank more often and hit more enemies in your AOE. This can also be used to help allies escape from bad situations and to help peel for your ADC's.

I max this whenever possible like all ultimates, the cooldown and the damage increases per level. If you want you can leave this ability for last, as it is a pretty bad ability for scaling. I am hoping they change this ability to make it stun for a less time in the early levels and stun for a longer time in the later levels.
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BFFs (PASSIVE)
Whenever Harley executes a skill, she gains 10% (+1% per 333 Bonus Will) Attack Speed and 10% (+1% per 333 Bonus Will) Decaying Move Speed for 3s. If an ally is nearby, the ally also receives the boosts.
Decaying Move Speed: 10% > 12.5% > 15% > 17.5%
Attack Speed: 10% > 12.5% > 15% > 17.5%

Great passive that scales with WILL, hell yes. With a full build, you will have a bonus of around +7% Attack Speed and +7% Decaying Movement Speed which makes a total of 24.5% bonus Attack Speed and 24.5% bonus Movement Speed at the maximum level. This passive allows your ADC's to destroy even more because of the attack speed and also allows your allies to escape bad situations because of the movement speed, nothing much more to say here.

I max this last, your Will should boost your Attack Speed and Movement Speed gains enough until the late game. 10% base stats are much better for the early game in my opinion, more worth levelling your damaging and healing abilities.
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Stolen Powers
Arcane Green Lantern's Healing Wave
This is one of the best stolen powers for a support in the game in my opinion. This power restores the same amount of Health as the normal healing wave, but it also increases all affected allies Attack Armor and Power Armor by 8 for 3 seconds. This will allow all teammates to survive a little longer in a fight, heck it can even turn the tides of a loosing fight sometimes. The Heal, the Attack Armor and the Power Armor increases per level.
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Gaslight Batman's Surveillance Camera
This is a great support item and I recommend it to be your second stolen power. Wards save so many lives in MOBA games it is unbelievable, and this stolen power acts as INFINITE wards and you can restock them which is epic. You can carry 3 cameras and 2 can be placed at once. These cameras cannot be seen after a few seconds of placement and last for 180 seconds which is amazing, that should be plenty of time to lane for a while and recall to your base for a restock and items. Also these cameras have increased sight range than the normal cameras which helps a little for seeing enemies sooner.
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Augments
I focus on defensive Augments, because this build does not include much defence. Once you have your Eye Of Ekron you should be doing lots of damage with your abilities.
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Tsunami
+24.06 Health (G)
+2.32 Move Speed (B)

This augment provides Harley with some early game Health, this allows her to survive a little longer in lane. The movement speed helps a little bit for mobility during the early game, dodging skill shots and roaming a little faster.
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Landslide
+3.01 Health/lvl (Max 57.19) (G)
+2.32 Move Speed (B)

This augment is very similar to Tsunami, but instead gives a weaker bonus early game and a stronger bonus late game. It gives Harley some late game health which is beneficial for surviving longer in the late game. The movement speed remains unchanged however and still only provides a little mobility and roaming speed.
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Fortress
+3.9 Attack Armor (S)
+3.9 Power Armor (S)

Being squishy Harley requires some defence, and not just damage. This augment provides Attack Armor and Power Armor, it is amazing for the early game as it will reduce all the damage you receive by a small amount. It works well with the bonus Health from Tsunami. It is very helpful as you will not build much Attack Armor or Power Armor until later in the game.
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Phalanx
+0.39 Attack Armor/lvl (Max 6.08) (S)
+0.32 Power Armor/lvl (Max 6.08) (S)

Very similar to Fortress but it is weaker early game and stronger late game. It provides Harley with some late game defence against both Attack Damage and Power Damage, it also scales well with the bonus health from Landslide. Even with defensive items Harley can be killed quite quickly by a strong team, this acts as a backup and allows her to take even more hits than normal.
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Core Items
Modular Tektite Vial (3)
This is extra sustain in the lane, sure you have a heal and the lantern but your Will is still going to run out very fast. This will recover both your Health and your Will, and I love it. Since Harley runs out of Will fast early, having some will sustain is very VERY helpful, the Health is mainly a bonus so you don't have to use Will to heal it off. Very cost efficient for the early game, you should sell it after you complete your lantern as you should have plenty of Will at that point. Keep it if you are still running out of Will fast.
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Abin Sur's Lantern (4)
This item is core to your build, it has great base stats, giving you lots of Will, some Power and Cooldown Reduction. It also allows all allies nearby to regenerate a small amount of extra Will which helps a lot for will sustain. It will eventually give you more will overtime and when you reach the 400 Will limit you get 10% Cooldown Reduction, that is 20% Cooldown Reduction and 800 Will in one item which means that you MUST BUY THIS WITH NO EXCEPTIONS. Leaving off this item is like having Harley Quinn without Mr J :(. I don't mod this item because you will usually have full stacks nearing end game anyway and you will have a chance to use it. This item also increases the Will Regen of nearby allies by 25%, this is very helpful when laning with a partner, this passive is in effect on the (1) buy, which is amazing for you and your partners early game.
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Gorilla Grodd's Helmet (4)
Another core item, this item provides 400 Will which is huge. It also provides 10% Cooldown Reduction and 15% Resilience INCLUDING a +15% resilience aura to nearby allies meaning you will have a total of 30% resilience. Resilience is basically crowd control reduction, -15% CC reduction for all nearby allies and 30% for yourself which is very helpful. It has an active which allows you to shield one ally, which is perfect in a 2v2 lane scenario for engagements. The shield active also gets boosted by your level and your WILL, this will make the shield block around 328 damage for one champion at full build. The shielded target also ignores DISABLES whilst it is up, another good reason to use it on your ADC to stop them from being immobilized.
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Eye Of Ekron (4)
I will be honest, I never used to use this but I have realized now how strong it can be. This item turns 2% of your Will into Power which is amazingly amazing :P. It gives some Movement Speed, lots of Will and lots of Power which are all beneficial stats for Harley making it a core item. Your final amount of Bonus Will should be around 2300, 2% of this is 46 Power.
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Steel's Breastplate (3)
This item gives you DEFENSIVE stats whilst also improving your OFFENSIVE stats, it is just too good to leave off. Bonus health, Will and Attack Armor is going to make Harley happy. This will help deal with ADC's and survive against them a little longer. The passive basically makes 20% of the damage you receive take your Will instead, this is very neat and should be considered. Think of it, an ADC doing 500 damage per hit and you activate this making them deal only 400 instead which helps a bunch. You also get focused too which makes it the perfect active for being an off-tank for 5 seconds.
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Situational Core
Amulet Of Isis (3)
Another core item, it has a good amount of Will included. This item gives some Movement Speed which is hard to notice, but is always welcome. You will also get 200 Health and 10% Cooldown Reduction, all of these are beneficial stats to Harley which is why it is a core item. Its active increases the Movement Speed of all allies nearby including yourself by 25% which allows for quick escapes, combine this with the BFFS passive for a whopping 49.5% Movement Speed boost (at full build and max level).

If someone else has built this item, consider a different item below (It is best built with Harley however because it combines with the BFF's passive)
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Offensive Items
Atlantean Royal Seal (4)
This item is great for offensive purposes. It provides 300 Health and 50 Power Damage which really helps. The passive causes enemies to receive a 20% slow for 1.5s when they are hit by your abilities, 10% for multi-target skills which counts as your Q and your R. It helps when catching up to enemies that are retreating from a fight.

A good time to buy this is against teams that have lots of Movement Speed and keep getting away from fights, it aids in catching up to them in order to get kills.
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Pandora's Box (4)
This item is full of useful stats, it gives you some Move Speed, Will Regeneration (which really helps), 60 Power Damage and 15% Cooldown Reduction. These are amazing stats, it will improve your damage and allow you to spam them more without running out of Will. The active releases a spirit that heads towards an enemy champion, once it gets to the Champion he/she will be slowed and revealed. This active helps when catching someone who is fleeing from a fight and even for just vision.

The best time to buy this is if someone has already bought an Amulet of Isis, it will provide more beneficial stats and will allow you to seek enemies out that may be lurking around, like enemy assassins.
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Cosmic Staff (3)
This is the big damage item, it provides 70 Power Damage and 10% Cooldown Reduction. It will make your Abilities REALLY HURT. The passive makes your skills deal a bonus 2% of the targets current health as Power Damage each second for 4 seconds. The passive basically makes your hammer mash Health stacked Champions. The second passive makes your abilities reduce the healing received on hit Champions by 50% for 2 seconds, this mashes support Champions as well. If the champion is below 50% Health then the debuff lasts for 4 seconds instead, your full build Q will have a lower cooldown than this meaning you can keep this debuff on enemy Champions for a whole fight.

The best time to use this item is if the enemy has lots of heals and support Champions, it will deny the regeneration of enemy Champions during a fight. Make sure you don't take this with the Amulet Of Isis as the Cooldown Reduction will be a wasted stat.
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Defensive Items
Kryptonian War Armor (3) (My recommended item)
This item is also full of beneficial stats. It gives no Will but it is purely for defensive purposes anyway. It gives lots of Movement Speed, Health, Health Regen and Power Armor. Again, all beneficial stats for Harley. The main reason I like this item is the 20% bonus healing received, your max level and full build heal should heal you for 432 health with this item. And you will heal that much every 6 seconds, very useful in my opinion especially for 1v1 fights.

Best taken when you are taking damage over time and need more healing to sustain that damage. It also helps when you get into lots of 1v1 skirmishes.
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Entropy Aegis (3)
This is a great defensive item and it helps so much against burst Champions. It gives 250 Health and 40 Power Armor. The passive grants a Skill Shield that ignores the damage and CC of the next enemy spell, it has a 60 second cooldown once popped. Again, it will deny one enemy skill and it will prevent you taking lots of burst damage, especially combination skills.

The best time to take this item is if you keep getting bursted down by power Champions, it will ignore their skills and prevent some serious burst damage.
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Ra's al Ghul's Robe (2)
I would not really recommend this but it has some great defensive stats which is impossible for me to ignore. It gives 45 Attack Armor and 30 Power Armor, this is a great defensive increase for one item. Its active gives you a shield when you die and sets your Attack Armor to 45 and your Power Armor to 30, if the shield is not destroyed you will revive with how much the shield has left.

The best time to take this item is if you keep getting bursted down by assassin Champions, it provides more Attack Armor while you are dead and because the assassin used all skills to burst you down, the assassin should take longer taking the shield down.
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Modifiers
Enhanced Gorilla Grodd's Helmet
+200 Will (Tier 3)

More Will is always welcome, as it will increase your overall DPS. It will also allow you to use more abilities before you run out of Will. This will also scale with your Eye Of Ekron. An alternative is the Improved Gorilla City Aura, which allows all allies in range to have CC removed a little faster.
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Empowered Eye Of Ekron
Will Force 2: Grants Power Damage equal to 4% Bonus Will (Tier 4)

This is one of my favourite Modifiers. The power you get from your Will will DOUBLE. Instead of gaining 46 Power Damage at full build, you will gain 92 Power Damage. This will make your Abilities hurt much more and it will give your Q, W and R more damage. This Modifier is too good for any alternative.
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Enhanced Steel's Breastplate
+200 Will (Tier 3)

Again, more Will is very welcome in our build. It will increase your DPS and will allow you to use more abilities before you run out of Will. It also scales with the Eye of Ekron. I would not recommend an alternative Modifier for this item.
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Thank you for reading my guide, if there is any problems with it you notice or if you have suggestions for improvement then do ask. Don't run away from Harley in the meantime. :D