I think i figured o how to kill Griffith/Femto. During Guts' rematch with Zodd at the hill of swords he Griffith said, "Beating... Faintly, high and shrill... In the blood... I thought was frozen. Is this... My vessel, the baby I took... Are these his thoughts?" I believe that was a clue. The corrupted child that was created when Femto defiled Casca, he is the key to all of this. I think Femto is somehow connected to it. I think that if you kill the child then Griffith will die too. Or the child contains the power needed to kill Griffith and maybe the God Hand as well. This is just a theory but i believe it to be a key factor.
ps. i have other fanfics that i am hoping others will take and do for me please.
rest in peace Kentaro Miura.