Enrage mechanics from a logical standpoint is kind of weird (if they can use this one shot attack to instantly end the fight, why don't they just use it from the start???),
This is pretty much why I've never liked them.

Then again, in video game land, we also liberally employ similarly silly things such as the Tactical Suicide Boss, stuff like Amon conveniently providing the ice blocks to hide behind right before he starts charging up Curtain Call, so I guess you have to suspend disbelief with things like this.