/At least have some earnable achievement left to do. Prepare one for an easy trigger.
/Go to Documents>Marvel's Avengers and back up the files, you never know...
/Disable your Ethernet.
/Go to Steam > offline mode.
/Go to Pc settings and change the date to before april 1, 2023.
/Start the game and play to unlock such achievement.*

*Sometimes Steam don't show the achievement pop up, you can check going to iron man and see if the skin is unlocked.

After that just equip it, play a mission to save it, then close the game and change back the date of your pc.

Enjoy!