Tips for Beating Possesor Boss
Dots. Burn, bleed, poison on the boss at all times. A cleric’s blackout neuter the boss pretty hard.
With a tank Aegis, use cover on whoever is your strongest nuker to gather up all those debuffs.
The extend buff power will also increase the time on those death tics giving you more time to remove.
Crystal form will clear them, but might leave your nukers without cover until it wears off.
If your tank’s second ability is monk, you should be able to just keep clearing the debuff with little issue.