the way you beat magneto with thanos is playing what thanos hates keep-away. with thanos keep jumping up and fierce, the fierce has some reach and keep magneto limited. when jumping keeping calling ironman when his unibeam assist. Ironman should provide some cover this is only to keep the assisting characters at bay.
The next thing magneto might do is rush in and do an assist to just to stop you doing that. this is when you attack when he rushs with the assist then tries to tri-jump use psylocke. when magneto tries to hit you in the air and his assisting character just jumped out . use psylocke to lift him in to the air and do a gauntlet power underneath him. the gauntlet power will push him back in to the other corner. after this the gauntlet power should do about 40% of damage. Watch the opponent close he might change his character punish him if he does. go back to jumping and calling ironman. if he gets close to you, walk up and just throw him. try to throw him in the corner you might get a bonus combo out of it.
if magneto is just holding you down with air dashing air combos then going in standard ground combos just block effectively. this is rare but if he just rushes in blindly with no assist, all you have to do is roundhouse then gauntlet power. when you have opportunies to kick to crap out of him you must do the most damaging combo which thanos has a couple 100% combos. if there is a chance you are Ironman and you still have thanos. jump up and throw smart bombs and capture type with thanos. when you land throw a jab unibeam and do it again. when its gets predictable stop. when he’s rushing you try to push block after the assist is over. if you ever miss a gauntlet power after psylocke’s assist and rolls if he didn’t roll behind you are in good shape. you can block before he get to you. in defensive situations you can use the gauntlet soul but be careful. Magneto can mess up and he can still kill you, and you can’t mess up once.