I play Warlock and also have a Death Knight.
Pretty easy to do it on those guys too. DK especially.
I've been doing it on my wife's lock as well. I just beat the shit out of everyone with my pet out, do it again when they're rezz'd (using the talent that lets you hit multiple enemies. Havoc, I think it is) and then when it's just Kael I dismiss the pet so he doesn't get Mind controlled. Going into Kael's first phase I try to make sure I'm topped off on embers. DPS him until he hits his second phase, and then summon the pet again and blow him up.
At least for me, I take out the chick mage and the guy with the shield first when they come back up. I face towards the side wall, that way if I'm kicked by the dude who walks, I don't go far. I just bounce off the wall and can keep DPSing. After that, the rest are cake.
With the DK, the easiest way to do it, for me at least, was in blood. I can do it in unholy, but I like using death strike and getting the blood shield in case I get stun locked. For my DK I actually kill all of them on the platform. I don't do that shit where I walk them around the room. I have conversion spec'd as my 5th talent, and this is what makes the fight so easy. Conversion gives you a heal over time based on how much runic power you have. Right before everyone is resurrected I make sure my runic power is topped off, and pop conversion right as everyone is resurrected. This insures that even if I'm hit and stunned, I'm constantly regenerating health. When they're about to resurrect, I cast Army, drop death and decay (since they're all clustered together) then I apply my diseases and immediately blood boil (I have the glyph that spreads diseases). This way each boss has something ticking on them, death and decay is hitting them, as well as the zombies beating the shit out of them so even if I'm stunned they're getting hit. Conversion is going in case things get bad, and when I come up I can immediately death strike one of them, and keep it moving. It makes the fight stupidly easy.
If you want to make it even easier, when the fight starts, let the first guy follow you all the way to the left hand corner. Since he walks slow as shit, it's one more person you don't have to deal with immediately.
Let me know if those strats work for you. Good luck.