You want to kill the pillars as close as possible to minimize the number of adds. What we do is have the dps assigned to the energy charges that goes to that pillar kill the pillar. Any dot classes should be dotting all of them on that side, remember the adds can be CC'd and as you move to group up waiting for the floor you want to single target and burst them down. Once the floor is back have the melee aoe the adds that are on elegon, and have the ranged dps the adds on the tank not actively tanking elegon. In phase 3 you want to stack them up and aoe them down before going all out on elegon himself.
Basically every pillar when it dies gives him 10% casting speed which causes the adds to come 10% faster per pillar, so if you have one pillar lacking behind you will have a period where he has 50% increase casting speed which means quite a few more adds.