The build I'm using (for Diablo exclusively) is this http://us.battle.net/d3/en/calculator/demon-hunter#aRXlVT!TYe!YcZccZ
Being a glass cannon, Smoke Screen with Lingering Fog is a must, although nerfed, it provides the best way to dodge those one shots.
Preparation is a must as well, you ARE going to run into situations where he will port or do an extra swipe that you cannot dodge, so an instant discipline restore is awesome, with a nifty 30% of not even causing a cooldown, for those extra ^-*! ups.
Now Rain of Vengeance+Flying strike is SOLELY used for the Shadow Clone in phase 2, since diablo moves around a lot and I haven't really tried using it on him. Don't need to anyway.
The bat companion is there to keep your hatred up at all times while you spam nether tentacles whenever you can.
The three passive abilities are basically your bead and butter ones, I've tried thinking of anything else, but you purely need damage on this fight, hardly any mobility or slowing or anything of that nature.
Sharpshooter is particularly effective between phases (Don't skip custscenes, your stats regenerate while Diablo babbles, that means if your discipline/hatred are empty, or you have a CD on Preparation, they will be restored by the end of the cutscene, and your sharpshooter should have your crit chance ramped up to 100%)
About the boss fight. First, ideally you start off by skipping the cutscene EXACTLY when he starts dropping down from his hovering animation. When you do it correctly, the cutscene skips but he still does his RP stomp on the ground, in which time you can get off about 10+ nether tentacles, which if you already have a substantial amount of dps, is quite a lot.
From there you watch for his charge-melee attacks. You constantly kite-and-shoot nether tentacles to him as he charges towards you. The charges can easily be dodged. Just remember where you were when he starts charging and as he reaches your spot you need to be PERPENDICULAR on his location. His melee range has a huge frontal cone for some reason, so as long as you are not anywhere in front of him, you are safe. If for any reason you feel like you are going to fail to dodge the attack, Some Screen asap.
The single most dangerous thing after his melee attack is his fireball. Make sure that you are atleast 20-30 yards away from him when he starts casting it (Raising his hands up with fire) and as soon as it starts traveling, Smoke Screen to prevent it from exploding if it's travelling near you. Or you can try to dodge, but that's real risky. In case of failure you can still smoke screen inside the fireball AOE but you have to time your potion just after your SS ends so you take a single tick of the dot, then SS once more to prevent the second tick. But usually failure means death.
The rest of his abilities are really easy to dodge/smoke screen through, so moving on to Phase two.
As I said, DO NOT SKIP THE CUTSCENE. It helps you recharge your hatred/discipline and cooldowns, as well as SS crit chance.
When he spawns the shadow clone, SS immediately and use the Flying Strike ontop of the shadow clone's spawn spot, in the mean time you should be atleast 20-30 yards away from it, after making sure you have the distance just strafe-spam nether tentacles and make sure you are in SS every time you spam them, the first clone should die pretty quickly due to Sharpshooter. After you kill the clone use Smoke Screen immediately and wait for Diablo to spawn, he usually spawns on top of you and kills you, so it's best if you are prepared. (You must use Preparation if you are low on Discipline)
From there go near one of the cracks on the ground and just kite him around it, shooting hungering arrows if he is not stationary and tentacles if he is just sitting around. Every time he ports - if he is 20 yards+ away don't SS, just run away, if he is on top of you SS and run.
The next shadow clone is the same as the first, spawn Flying strike with SS as it spawns, but this time you must be extremely careful and make sure the clone is stunned before staying on one place. You must always be moving around/strafing because it's a fast one-hit kill.
This is basically it for phase 2, also watch the fireballs, avoid them with SS+preemptive range at all costs, because that crack you are kiting on is sort of unstable and fireballs tend to explode prematurely there.
If you've reached this phase, then you've practically won already. The only thing you have to watch in addition to Phase one's bull@#$% is the Fire breath or Incinerate or whatever.
So, make sure you are positioned somewhere near one of those two big crystals so that every time he casts it, you stand behind them. Fireballs/Fire Breath/Fire nova do not penetrate those crystals, so it's a safe net, but also a death trap since he teleports if you are there for too long. So keep an eye out and be ready for SS.
It took me about 2 days of practice and dying, but I managed. I can say Belial was a lot harder than this fight, atleast for me.
Any criticism or questions are welcome.
My character details
Got about 46k dps with no Sharpshooter on.