Jungling Nightmare Batman

Starting Items - Start with Jokers Crowbar (1) and 5 Health potions. On your first back upgrade Jokers Crowbar to start stacking the AD as soon as possible.

Early Game - While farming up in the jungle and ganking lanes try to save an average of 800g so when you back you can buy an upgrade of Soultaker Katana and a health potion. Try to buy Soultaker Katana(3) as quickly as you can to start stacking the bonus AD and Lifesteal. After rank 3 of Soultaker you can either opt for Cheetahs Claw(4) or upgrade Soultaker Katana to rank 4 then go for Cheetahs claw.

Mid game - After you finish with your first 3 core items start rushing Joe Chill's Revolver (4) for the bust of AD and Crit. Joe Chill's goes great with the amount of lifesteal from Katana and Cheetah's claw, plus the attack speed from Cheetah's claw. After finish with Joe Chill's rank 4 invest in to buying and maxing Velocity 9 Implants for the extra attack speed and crit, the movement speed also goes very well with Nighmare Bats Q slow, plus E movement steroid.

Late game - Once the core 5 items are completed consider buying Ra's al Ghuls for the Attack armor and Power armor. Rank 2 of Ra's will also give you a revive passive, which is very useful when team fighting, or when trying to take on multiple enemys like the bad*** you are. If your game has lasted long enough, or you're so fed so fast, consider selling Cheetah's Claw for Batman's Utility Belt which provides great sustain to Nightmare Batman with the Health, Attack Speed, and CDR that it provides, not to mention the Purge active effect.

When jungling always try to use W when starting camps as it will provide good energy regen.

When fighting enemy's use E when close enough to catch them and auto them so the duration will reset.
Spam Q for the high damage output and the slow.
When thinking of ulting (R) attempt to throw your meteor where they are standing, then lock them in place under it with your ult so that the meteor hits them providing maximum damage output for nuking someone from 100% to 0. If they survive your meteor + R combo predict what direction they will run and mash your Q so that you will hit them with it the moment your ult stops for the extra damage and slow to finish them off.

                                         Laning Nightmare Batman

Don't. Just... don't.