I keep getting my ass kicked on Act II: Blood and Sand. What a stupid quest.

We shall not go to Caldeum, tis a silly place.

What class are you? Pump Vitality if you think that your char is too fragile and try to get +X to health per kill or per Hit.

I hated those with a passion. I just started Act II and at level 15 Witch Doctor I'm finding that my beloved Rabid Dogs are pretty much worthless now :(

I always used the dogs. They deal little to no damage, but they're only there to draw fire away from you on mobs, since you have no crowd control as a WD. Also, your Gargantuan will be the main minion, once you unlock it, so don't worry too much.

There's an amazing rune for the dogs also, Life Link, in which they absorb 10% of the damage dealt to you, so that basically means a 10% Damage Reduction and you just can't have enough of that.

The succubus curse kinda sucks, you can kill yourself pretty quickly with high damage output. And some of the enemies' AoE spells and abilities cause some real damage and can be a bitch when paired with jailers.

You're already at Act III Nightmare? Damn you move fast xD.

I just started Act I Nightmare. Diablo was a bit of a challenge for my WD, mostly because he seems to aggro on my position and forgets all about my minions (plus he can't be slowed, which makes half of my skill set useless against him :P). Nothing major though, managed to beat him and used only two potions. 

No, only played through Act III on normal! Diablo was pretty easy as a Demon Hunter, I just vault around, deploy sentry, use the volley skill for heavy damage and pepper the poor bastard with Rapid Fire. He has a lot of hit points though, took a lot longer to kill than anything else in the game.

Do you use any of the +% movement speed Passives? One thing that sucks as a WD is that you don't have enough speed to run away from most of Diablo's attacks, especially if you move too late. I hope to find a pair of good boots that will have +Int and +% movement on Nightmare before tackling him again xD.

Also, it seems that Demon Hunter is being considered the Boss Killer class. A lot of people in General Chat ask around for DHs to do bosses. I had to help around a couple of friends that are WDs to beat Azmodan and Diablo, because they weren't able to do it solo. 

Demon Hunter.

Well then, I believe Mummelman could give better tips on how to tackle that quest as a Demon Hunter. I'm still going to try Demon Hunter later today, but i'll be nowhere near that quest. My tip is a generalistic one that helps out most characters, since people usually forget to pump vitality for survivability.