After thinking about it, I've come to a few conclusions as to how Erik wins.
1. Magneto can controle the iron in GR's blood and rip him apart...wait a minute...
2. Even if his chain's aren't metal, Erik can find something like a chain, wrap it around GR's neck and strangle him to death...no that doesn't sound right.
3. Erik uses the metal around him and summons a metallic death ball to crush GR. He'll feel a whole bunch of pain after that I think...I hope...maybe 😬