The Prisoner Boss Act 4 Bugged

Anytime he is down and I pull the lever to shoot an arrow, his entire HP regens again regardless of him being hit with the arrow.

Im using a freeze build, so not sure if that is a because this animation doesn't work in comination with him being frozen.

but it means the fight is literally impossible

EDIT;

okay did it for another try and the first ballista didn't work, but afterwards the last 2 did work.