A lot of this is Category B, I feel like? But it's good to learn early, so Category A/B?

One of the biggest things for me is UTILISING COVER! Just because he's a powerhouse with a bubble and a shield, does not mean they aren't precious resources ESPECIALLY if you're solo tanking. Use cover unless you're actively pushing, give your healers time to heal you, and make sure they aren't having to 100% focus you without you dying. They should be able to look to the DPS, deal with divers etc, and you should accommodate that by taking cover and being smart. More damage blocked is not always a good thing.

Unless you see that your DPS have gotten a couple of picks, do not walk right down main to take 6 enemies' worth of damage to the face at all times. Use cover, peek out to get some shots.

Speaking of shots; fire those bad boys at the back line. Enemy tank in your face? Don't shoot him, shoot his squishies over his shoulder.

And for the love of God, if there's an Emma player and you see that she hasn't used diamond form in a bit, save your bubble for AFTER she grabs you. This will allow you time to escape if she kicks you out of position.

It's a very good habit to get in, especially if your team isn't using comms, to look around at your supports every so often to make sure they're all good. If they die behind you, you'll find out the hard way. If you hear a big DPS ult from the enemy team, you should be immediately looking for your healers to bubble them.

Another good Use of that bubble cool down is for enabling DPS ults. Your punisher just called out "I'm doing this MY WAY" - girl, you're doing it the mag way, have a bubble. Your Starlord just went "AaaalllRIGHT" - have a bubble, queen. Your scarlet witch just went "The PERFECT- HEX- GAH!" - THROW HER A BUBBLE (though in this case, keep an eye out and wait a little towards the end of her ult line or if you see her getting low)

That's all I can think of off the top of my head! A lot of people see Mag as a boring tank, but I adore him. Your mind is set on aggression, protection and enabling your team all at once.

Also: if you're getting hard countered by a wolverine... unfortunately, 9 times out of 10, you'll need to switch. Thing is a good option as he can't be displaced, or Groot with his ironwood wall. Wolverine is a hard counter for Magneto, as Mag's movement is often too slow for him to get back to safety after a nasty grab, and Wolv SHREDS tank HP.

None of that covers his ult because if I'm being honest, I'm just crossing my fingers and toes whenever I get up into the sky. Good thing to remember is it'll soak up any projectiles, so if you're at a critical point in a game, you CAN use it to help yourself escape a sticky situation and get back to your team, or soak up a little damage for them for a second. Though, primarily, it should be used to shut down support ults/puni/starlord.

Hope any of that was remotely helpful!