Ivy main over here. I have been playing Ivy since day one when I got into the IC closed beta and have not stopped since. She is my most played champion out of all of the champions in the game. She is simply awesome and I love her kit. I consider myself a pretty good Ivy player and I want to share what I do with Ivy that makes her work for me. 
I play her in both top and bot in GH. 

The first thing you most likely noticed was my build. I do not play Ivy as a straight PD mage or a gambit aura support. I play her has a tanky PD champion with max CDR. I find that this is the best and most effective playing style for Ivy to maximize her potential. Sure you will deal less damage then a PD ivy build, but look at all the advantages from this build...

1. If you reach end game you're very tanky and hard to take down due to the stats you have and the abilities in your kit. This lets you stay alive longer to protect your allies and disrupt your enemies. 

2. With max CDR Root Strike, Blow Kiss, and Thorn Shield are up very often which helps proc her passive and increase your survivability greatly.

3. This build makes you a decent damage threat in teamfights. Think of yourself as a consistent damage mage.

One thing to remember with Ivy is she is a little bit on the weak side for her early game, but once you reach mid and have at least 3 items it should get better real fast.

Amps and Mods: Amps I suggest getting a 10% CDR from amps seeing that the build only gives you 30%. For Mods I use Empowered Kryptonian Armor, Empowered Atlantean Seal, and Accelerated Abin Sur's Lantern.

If im using Bleed Portal I use Accelerated Abin Sur's Lantern, Empowered Atlantean Seal, and Altered Bleed Portal.

Stolen Powers: If I am bot I run what you see in the guide for stolen power. If I am top I usually take Gaslight Batman's Cams and either healing wave or Atomic Ivy's X-Ray vision to sweep wards. If you take BP you need to take Meteor Drop.

My starting items are usually Hawkman's Harness (2), Abin Sur's Lantern (1), and Modular Tektite Vial (2). Once you reach 1k gold recall and upgrade Hawkman's Harness to tier 4, then sell it and buy Abin Sur's Lantern (2). Once you have Abin Sur's Lantern (2) start working in getting Boosters Gold's Power Suit then after that get your Abin Sur's Lantern to tier 4. After you have these two items you should have 20% CDR and an additional 20% from amps and Abin Sur's passive later in the game. After that I usually get Royal Seal next for the increase health, the slow, and some Power Damage. Then Kryptonian War Armor, and Pen.

Now that leaves you with one more item you can get. If you are vsing a heavy AD team Helm of Fate is recommended or if it's a heavy PD team Entropy Aegis. If it's a balance team I would just get Gorilla Grodd's Helmet for the passve and aura. You also always have the option to get Bleed Portal if you are top.

That pretty much my build. Though here are some tips you want to remember.

1. When you're in base at the beginning of the game spam your spells to get your passive proc up to 4, so when you cast a spell in your first fight you get your passive up. You should also do this when you recall if you have the time while your health/mana refills.

2. To use your passive to the best you can, make sure you are always in a good position to heal as many allies as you can or make sure you know you and/or your allies can stay in it. Keep an eye on when it's going to come up.

3. If you want to shut down a ADC or even a mage for a decent period of time chain your Root Strike, Blow Kiss, and Thorn Shield together. When they get knocked up and recover, Blow Kiss them to reduce incoming AA damage (if it's ADC), then Thorn Shield yourself.

4. It's usually best to use your Deadly Flora right after you knock someone up or if you have someone on your team that can make the enemy team stay in it. You can also use her Deadly Flora has a disengage tool.

5. Once you are tanky, don't be afraid to make the enemy team attack you. If they are busy trying to kill you, your ADCs/Mages should be cleaning them up. If they ignore you then focus on keeping the target alive they are going for.

While this is a GH guide, I do this build in GD/CC too and it works just as well. I hope you guys/girls find this guide helpful and I look forward to playing against you all in-game.