Hey guys this is my own video and I hope it helps.
Simple enough, all you need to do is beat the game then after the final mission you are on permanent free play mode in the city. I did this right after the credit role. The main thing here is that you need to hold down ( Y ) to switch characters to the Hulk and they tap ( Y ) to switch to the second and hold ( Y ) to choose Loki. From then on all you have to do is keep pressing ( X ) until the hulk grabs Loki, and slams him back in forth like in the movie. Then "Puny God" is yours.
Edit: Thank You
destinee ehgc for pointing out that if you stand still next to Loki and press ( X ) he will preform the move much easier.