When Vexallus spawns each Pure Energy you kill them and get a debuff that stacks and puts a dot on you, that dot ticks for more and more as the debuff stacks. Well, If you are doing him with 2 ranged, have 1 stand on under the "candle fixture" on the left and the other on the right, when standing here your ranged should take out the 2 adds and the debuff stated earlier cannot stack more than 1 time, therefore, no rediculous group damage.
http://upload.wowhead.com/images/scr...ized/78095.jpg
those are the candle models on the right and left, you dont want to stand under those though, you want to stand on the ones farther back from him which arent in the SS