You get a debuff if you drop below 10k health
Then there is finkles mixture, a buff that prevents you drom dropping below 1hp when you had 10k+ before, which occasionally fades for a few seconds when the bot is offline.
And of course there is the final phase below 20% hp, where no healing is possible anymore.
primarily, all healers are damagedealers in this phase, trying to burn him downbefore your raid dies.
i do a few things in that phase though (holy paladin dps is not comparable to a holy (disziplin) priest doing dmg.
You can use hand of protection on chimaerons primary target when it is about to die, so he drops target.
I havent tried if we have something to taunt him and use divine shield for 8 seconds too.
I would also use light of dawn and holy radiance during the system shutdown where everybuddy hugs together.