Prisoner boss a4 broken

no matter when we use balista it regenerate to full hp. fix that please

The boss is completely bugged. The ballista don't prevent him from healing, and the boss heals to 100% HP on minions. I can't beat the boss with this build. What kind of crap is this?

Same.
Minion build here.
Ballista does not stop his healing, and sometimes he full heals without the ballista message. I tried to kill him 3 times. All three times the ballistas failed.

Edit:
I reset the zone, slogged through all the trash mobs to the boss.
Tried it again. This time the ballista worked fine, easy kill.

Ballista timing is way broken. We've fought this boss 3 times in a row and he simply recharges after hit by a ballista shot.

We've tried: both party members firing ballista
Also tried: only one party member firing ballista

Still doesn't work -- will reset the instance and try one more time.

We reset the instance and the ballista shots still did not work.

We just had to power thru the fight until we won. We beat it as if never firing a single ballista once.

Not a fun experience -- which is not something you want to hear about a video game.