Pops adds at 75, 50, and 25%. Adds at 75% are cubes and can be stunned, snared, rooted, etc. Adds at 50% can NOT be stunned. Adds at 25% also can't be stunned (not sure about snared, but know they can't be rooted).
Basically, back everyone off as adds are going to pop, since they pop just behind the named by the stairs in a line and immediately rush forward. Setup tanks to intercept if you don't want your tank to be eaten. During adds phase you can summon tank the clock so as to reduce the healing needed (it's perma-rooted).
Easy peasy. The 4 rooms are easy too, if you have the order right and do your engage properly (don't rush in the room or you'll die, don't leash mobs out of the room or you'll die, etc). Send in a tank with riposte disc/deftdance up, have him draw agro, then send in sk's and offtanks to pick their target and peal off.