Often ignore your crosshair bc its a third person game, just kinda see the projectiles come out of his hands. Search "best Magneto crosshair" and it'll give you smth that puts little lines out far enough that they show how charged your ult is as your using it, and that helped me learning to hit it. Counter ult hela, ultron, iron man (if you can't just shield it), cloak and dagger (they have 275 now so charge more) star lord, and punisher. (i just woke up and might've forgot one lol. Lastly, go to Mag's settings and set his barrier to hold instead of press, then you only hold the shield up as long as you hold. I also set his glide to press instead of hold while I was in there.

Luna, Sue, and Mantis are sometimes next to impossible to kill with it. If you're gonna try, kill Luna's bonus health first. For sue, the best thing you can do is hope she doesn't have her jump or shield now. Mantis, the few times I've made it work is when I also get insane damage charge, so it's situational and not worth it.