When in the steel mill to fight Magneto there is a place you can stand and kind of hide till Magneto wants to work on taking his jail collar off.
From the start point run to your right like you are going to go up the stairs but by pass those stairs by turning right again.
You will see another set of stairs, go up those till you reach the first level.
When you do reach the first level turn left, watch out for the broken off part of the catwalk and head towards that corner that is ahead of you.
It has a little ceiling that when Magneto "grabs" you with his powers when you shake yourself free it won't hurt you as much when you drop down.
This corner is good to hide from his electric balls and all the metallic things that he hurls at you.
Watch him carefully, you will see him start to float down towards your left.
That's where you want to meet him, when it's time for him to try to get his collar off get out of the corner, turn left and there's a little drop off.
Jump off and you'll see Magneto hovering near a little platform.
Run up to him and slash him with your claws. FYI: When avoiding his electric balls and all the things metallic that he's throwing at you retract your claws so that the healing process can keep your health up.
It'll take a while for you to get used to this but it'll work, once I figured this out it took me only on try to do this....GOOD LUCK!