I'm Always Angry!G 20

Hulk is introduced in the first level, "Sand Central Station". All you need to do is press and hold the Y button and you will transform into Bruce Banner. Do not let go, but instead just keep holding it and you will continue to transform back and forth between the Hulk and Dr. Banner until the achievement pops. You can use a rubber band or something heavy to hold the button down for you so that you can go do something else and let the game do it for you.

You can also just wait and play through the game as you will have other opportunities to transform back and forth for puzzle solving as you play through levels.