for the first boss, melee dps will have to run out and bandage when they can to save themselves, it'll help with healer alot, he's not a terribly hard hitting boss, so fear not if the healer tries to top off others, not too big of a deal, i've never had that problem and i run a full melee team:
Prot warrior, fury warrior, fury warrior, rogue, priest.
On sepethrea it really is a matter of just not "KILL HER FAST!" but rather, move the elementals. Even if you just have to beat on her very VERY slowly, then so be it, the more ranged you have the better, but if you're a lock and you got an elemental coming for you, SCREW your dps meters, RUN, just keep them away from her, and the rest of your party, kite around in a big square around the room, and when they start to pulse, then go ahead and try to squeeze in your dots/nukes/ranged dps, or wahtever. This puts the most stress on the warrior who will need to taunt and grab her attention inbetween agro wipes as much as possible to prevent the boss from killing people. It also puts a ton of stress on the healer who will need to basically kite elementals if they're on them, while healing others. Just try not to cross paths, keep the kite rectangle going, and slowly burn her down.
as for the lords. well that's life, they hit like dump trucks, I die all the time, as long as the others don't, or we proceed through, its worth it to me =]