So... it's an Antaeus kind of situation? I wonder if the Heracles solution will work or if it'll be some kinda nonsense like F/SN Berserker's kill-him-so-hard-he-loses-multiple-lives.
[edit] Dunno why I get a "dumb" reaction. That's literally how both stories go.
In Greek mythology, Antaeus recovers by drawing strength from the earth, so Heracles has to lift him up and crush him without letting him touch the ground.
In Fate/Stay Night, despite Berserker having multiple lives remaining from his Noble Phantasm, Saber kills him with one attack from Excalibur. Given that it's not a sustained attack, it doesn't really make sense, but that's how the VN/anime went.