I like to think that the sweetest part is that Guts wont have to do a damn thing. He'll be on his last legs towards the end of their fight, before he is saved by the spirits of his fallen comrades, channeled through his son, and be able to match Griffith's demonic powers. Griffith wont be able to comprehend this new turn of events, he'll slowly fall into madness trying to hurt Guts, only for Guts to rebuff him at every turn and slowly knock the demon powers right out of him, leaving Griffith in the same state as he was before being revived by the god hand. After this Guts will prepare to deliver the final blow, and Griffith will have accepted his fate, only for Guts to stay his hand and sheath his sword.
He'll give a speech about how he has let go of his anger towards Griffith, how now he only feels pity for him, and refuses to finish him off. Griffith wont be able to accept this, he will only accept death if it comes at the hand of the only other man he respected, but Guts won't give a fuck, and he'll pimp off to his friends who are still alive, and reunited with a revived Cassca. The sight of Cassca and Guts sharing a genuine, tender kiss, of true love will Drive Griffith mad.
Now he'll seek to end his life by his own hand, but before he can stab himself in the chest, He'll feel the hand of his former fellow god hand Void. Void tells him that his failure to bring about the age of darkness or whatever really pissed off The Idea of Evil, and now he's gonna have to pay. Next thing you know Griffith is back in his old dungeon cell in the Midland kingdom. Void lets him know that he's not going to get such a nice little punishment as death, he's going to get something much worse, Life, eternal life that is. He'll be stuck as a mute invalid, stuck in a dark pit for the rest of eternity, without even the whispers of demons to comfort him, he'll be truly and utterly alone.
"Look on the bright side" Void says "This realm is all yours to rule"