Last boss gave me some trouble, finally got him down after ~7 or 8 attempts. Here's my tips:

1. Avoid the clouds above all. If your team is dpsing an add and the white swirly spawns, move them out and ignore the add. Nearly all the damage I was taking in phase 1 was from the white swirls.
2. Run to the middle with the last add. Gives you a better chance of not getting anyone knocked off.
3. After the bouncing around, just stand and dps. Make sure they're all facing the right direction, and move into healing range if need be. I'd move out of green swirls if they got to me, but once I got to this phase with all 5 still alive and up top, I won with no issues.