Is he the big lich guy? I've only done the instance once, the way I did it was to position all my guys on top of each other "behind" him against the wall, so adds that are streaming from the sides and fronts all go to one spot. No real issues burning them down, make sure to save taunt so if an elite spawns and heads to a shaman you taunt it off.

When he unleashes I just go all out on him, and chain heal often. I ignore the blizzards and AOEs and other random stuff, just chain heal through it. Perhaps you need a certain level of gear to not run OOM, for my shamans I have been picking up the spell power/mp5 gear over spell power/crit so they can last slightly longer -- but I also have a mage, and he gives everyone the int buff which helps.

The last boss is really fun as a multiboxer, all your guys get turned into skellies and you have to use one of 4 abilities -- not too hard though, as long as you have different key mappings for your tank and casters for your main bar. You don't want to be taunting the boss with the shaman