Round 1:
If the Death Star was laying on top of Earth. Let's say... On top of Europe... And Magneto was standing on Mount. Everest looking at the giant ball from the horizon, could Magneto use his power to lift the Death Star off of the ground and up into the atmosphere and back into orbit?
Round 2:
Magneto attacks the Death Star from within. All inner weapons and defenses locked onto him. Can Magneto get to the center and destroy the Death Star?
Round 3:
All turbo lazers locked onto him from outside of the Death Star (except main canon. main canon is off-line/). Can Magneto's shield hold them off while he concentrate all his power to crush the 160km (in diameter) and compress it into a size of a baseball? (AKA: Magneto has a power to increase gravitational forces to compress matter and it's called "Gravitational Squeeze")