This must be completed after the story, as that's when Loki is unlocked. Buy Loki, then find an area with few things to be destroyed. Make both on-screen characters Hulk and Loki. The only tricky part is that Hulk has several special moves, and the achievement only counts one specific one. If you've seen The Avengers, you know what it is; if not, it's the one where he grabs the enemy and slams them into the ground. Push forward and
into Loki until the special triggers.