On that first boss, you stand at the head of the wind to help avoid Necrosis. For instance, if the wind along the ground blows from the entrance of the boss arena, run to the entrance gate and watch for those balls of light around the edge. To the left or right of the gate is a spot to stand and you can avoid the AoE altogether. The second time it comes around, you have to stand directly in front of the boss at the head of the wind as well, and move to the spot behind you that's open, to the left or right of the entrance/exit gates of the arena.
On the last boss, I see a lot of tanks chasing the boss around the arena when the boss does a Prance. Best not to do that and instead go stand along the outer edge of the first prance circle where the boss will jump to on its third or four prance, so you can move when the first circle disappears and you don't get vulned/die. You can still do ranged damage until you can figure out how best to move with the boss if you choose to do so.