re: transition.

You can keep dpsing the boss until he becomes untargetable and there shouldn't be any danger of getting oneshot by the barofield/shield as long as you move out after. Just fight normally and rotate around the boss to get as close to the add spawning location as possible to minimise downtime.

If he transitions mid nerve gas - you just get more time to dps the boss. not a big deal - he'll still be targetable until the transition actually takes place.