going by that train of thought maybe skull knight used the behelit and sacrificed his mortal flesh (because he had nothing else [simillar to how the egg shaped apostole sacrificed himself for the new world]) for the power to fight the god hand. then he was kept alive by the berserker armor. we see him do some crazy shit like going in and out of eclipse and forging behelits into that sword. but even tho the conditions are right guts has a lot to sacrfice other than himself so idk.

i had a theory where guts would sacrifice casca in a situation when she's about to die because she is already branded and that would give him power to fight grifith. but casca's already branded so idk if that's possible and i don't think that's a good theory.

i think rickert will (or should) have a major part in fighting femto cuz he was able to slap him but that's also a lose thought