Might wanna not use the [petition] thingy, I don't think the devs are very fond of it.
But this is something that can be implemented surely, we have those fighting games where you're invincible and the boss is, too. So you can practice rotations and dodging while both you and the boss' health doesn't drop, and you can skip to phases.
Or when the boss dies, there's no cutscene, only goes back to the start of the encounter or even go back to 100% health..