For second boss, the skill(Salt Deflagration) does a high %hp hit = use serenstum
We tried it on normal a few days ago. Seems like salt has a very short range. We just got everyone to run away from boss, like you'd do for the second boss in WA when he pulls you in. The problem was the mobs... We lived through several Salts but mobs got us lol.
For first boss, a red flash on top of someone always happened before they got oneshot, so pop immunes if you get it. 1 person will get locked up by a cage, buy you can DPS the cage and get them out.