This requires you to perform all of the large transformations in the game. You will need to take control of each character listed below and then hold to transform them:
Hulk (Bruce Banner)
Red Hulk (General Ross)
Venom (Bigfig Venom)
Curt Connors (Lizard)
Stan Lee (Excelsior Hulk)
The achievement will unlock once you have performed all 5 transformations.