First group: Target the girl in the middle, she will always turn into a caster. You simply want to make sure you get agg on that one instead of trying to pull, as it will not move.

Second group: I run upstairs ahead of Arthas and set up immediately following the first group so that I know I'm ready. The group is a little more varied, but again look for the casters and make sure to keep agg on them. 4 appear in total, but you usually get at least 1 caster, 1 hunter, and 1 adversary (I think is the name). Take the hunters down second. Save the adversary for last, because they have an ability to reduce all damage they take, and take the longest to kill.

Third group: Set up outside of the hallway and let Arthas go in by himself. 2 appear at the far end of the hall, and 2 at your end right in front of you. Again, get those casters on you and be sure to pull off of Arthas before he can get waxed (he's a bad tank). Again, casters die first, then hunters, then adversaries.

Fourth (and final) group: Again, 4 spawn. Usually one caster. Agg the others and pull over to wherever the caster appears (all 4 appear in the middle of the large room). This is usually the biggest pain for me in the whole instance where I try to dance with the mobs trying to keep all of them in front of me (paladin tank, so I need shield blocks to reduce damage).

Boss: I just spread my toons out into a line and prepare. He appears by the table in the back of the room and talks briefly before the fight starts. Pretty much tank-n-spank, though he will occasionally do a time-warp effect where you are slowed for about 5 seconds and he rapidly ping-pongs around all your toons, damaging all of them. This is NOT an aggro wipe, and he will return to the tank when this is over. Group heals will get off really slow, so if your priest has Circle of Healing (instant cast group heal), you may want to fire that off in the middle of it.