I was doing this instance yesterday with my 3 DKs and 2 randoms being rogue and paladin healer. I was tanking and doing DPS. it was quite easy and smooth run, until we got to this boss and were eaten realy fast. The thing is he cast some sort of reflective shield, so you have to stop dps. Otherwise you will kill yourself :-),He also does some other abilities like heavy nuke for 18k dmg on heroic, which can be reflected back or interupted. Another ability is well of souls, you just need to not stand in it.

Hope this helps.