Stan's SoapboxG 20

You will need to rescue Stan Lee from every peril in the game before you can unlock him as a playable character (you can track this on the Pause menu).

Once you have unlocked Stan Lee (definitely the hardest part of this achievement), then simply take control of him in free roam and hold to transform him into Stan Hulk.

The achievement will unlock shortly after you've transformed him for the first time.