@jokerxhisoka
find a way, he can go back in time, breaking a prison should be more difficult than defying the laws of physics.
Try using your head? This is a novel / story, and solutions are never, ever as simple + easy as "just go cut off the BBEG's head before he gets powerful!"
I think it will roughly go like this:
Zeff tries to go get Graff's head cut off, doesn't work (obviously).
They try summoning Yamato to blow up the prison or something, doesn't work.
Eventually Zeff says 'fuck it' and asks them to attach the brain parasite thing to his head too.
Proceeds to spend 10,000 years showing the whippersnapper how to REALLY master magic effectively in your second life.
They come out of the mind based hyperbolic time chamber around the same time, have a climactic final battle.
After the battle, Zeff tells the girls that after 10,000 years of being alone he's realised that he really does love all four of them, turns them into a proper harem.
The end.
...or something like that.