That was the problem with them, there was no reason for a flash and the beam didn't fit the temple at all. I've tried making it shoot off the screen in an exaggerated blur. It's weird for a bouncer to inexplicably do that, but it's better than simply vanishing. I'll probably stick with this if we can't think of something better. (Sorry, for the lack of visuals. I want to keep as much of the boss battles a secret as I can. Here's the blurred blue bouncer...)
Hmmmm... well I'll brainstorm ideas, some may work others may be stupid or be completely undoable in engine but here goes:
-open up a trap door that he falls through.
-have some rocks or a pillar fall on him
-have whatever the boss is get angry at him at kill him using whatever it is he does
-have the bouncer get angrier and angrier and more bloated as time goes on (not sure if you can change enemies faces and looks and colors in KS+) before he explodes/pops/flies away
-have some spikes on the ceiling or wall and have him get impaled on them
-if he is teleporting around, have him accidentally teleport into a wall/water/fire/spikes