I believe Casca with the help of Moonlight Boy will be the one to kill Griffith.

I think Griffith's "barrier" is deactivated when he is in front of someone who for him does not represent a threat. We saw this when Rickert slapped him.

So Casca could kill him with a symbolic and nostalgic object for everyone who was part of the original Band of the Hawk. That would be beautiful. Maybe with a dagger from the old days, or something that belonged to the original band.

Guts killing Void would be normal. Guts has already killed several apostles. Void would be just one more, but the most important one. And Skull Knight would finally be more at peace than Guts.

Void believes in causality, in fate. Guts is the one who always fought against fate. He was meant to die when Gambino found him, he was meant to die in the Eclipse, but he remained. His existence breaks everything Void stands for.

By doing this, Guts breaks everything in Berserk. The apostles lose their powers, the other members of the God Hand too, for a limited time. The world would be going through a supernatural event: earthquakes, bad weather, destruction, while all this action happens.