Hey all. I was asked a lot from bnet friends how I made diablo so I thought it's maybe helpful for you too
Passive Abilities are your choice pretty much, but I've been using Sharpshooter, Steady Aim and Archery. You can probably drop Steady Aim for Perfectionist but I wouldn't change the others!
For offensive Abilites:
Rain of Vengeance (Flying Strike)
Smoke Screen (Lingering Fog)
Preparation (Backup Plan)
Marked for Death (Mortal Enemy)
Entangling Shot (Shock Collar)
Elemental Arrow (Nether Tentacles)
Phase 1:
Start from the very left or right, this will win you some time because he will get stuck for some seconds and then you should've made already enough damage for the cage phase. Just make sure that you dodge the single fire ball with Vanish (dont let it hit you, even if youre vanished so it doesnt pop a fire rune on the bottom!) and shoot him safely from behind the cages. If he teleports to you, vanish and get some distance and then slow him with Entangling Shot and kite him a bit. When he's far away, start shooting with Elemental Arrows again and make sure you're not close to a wall so he doesn't use teleport. Sometimes he will, use Smoke Screen then.
Phase 2:
Don't click the cinematics away so your cooldowns get back up and your sharpshooter stacks!
As soon as the phase starts, count to 2, then vanish and use Rain of Vengeance on your copy. If it's not down after the stun, use another Smoke Screen! Then Diablo will appear. Just kite and dodge him like in P1. Try to keep your discipline high for the copy phase and don't nuke him too hard if Rain of Vengeance is still on CD. Rinse and repeat
Phase 3:
Don't click the cinematics away so your cooldowns get back up and your sharpshooter stacks!
Get some distance because youre spawned in the middle now. Then just get him down like in P1. Just be very careful and run in a big circle and don't get to close to walls and kill him!