Just for a hint, when cornered, try and put space in between you and him when you can. Make sure you know how to use the pushback (Advancing Guard) feature to push him off you and fight some of the pressure game. Advancing guard really kills a rushdown strat from my experience, but I may not have been up against a lot of players who know how to do a proper rushdown. That’s a good general tactic you can use.
However, like AmishOpiate said, his guy sounds like he’s trying to learn the ins and outs of Magneto, which means he learns what moves Mags can use beats out other stuff. You don’t have to get technical or pro and learn frame data like that, but when you stick to certain characters and kinda learn to use them, you’ll get your game up. Random select is fun, but it won’t help you get better in general. Plus, I’m not going to put up the chance I might accidently get MODOK. Some people may like him, but he’s not my kind of character, and he’s usually gets beat out in a lot of instances.
So find a character, or characters you like, and learn to use them together. I love Dormammu, but am struggling to find a decent team to use him with, please I don’t think he’s the best lead character in most instances, but I am loving his projectile destroying medium attacks. Currently, I’m pairing him with Doom and either Iron Man, Wesker, or Akuma. Just find what works best for you. You’ll find something eventually.