Try playing the game again, with a focus on extracting one letter from each level.
Learning Box2D for this puzzle was a lot of programming work. But the instant Misha proposed "Aangry Birds" as a puzzle title, I knew I was going to do it.
During the hunt, one team solved the puzzle by viewing the source code and editing the 'currentLevel' variable to skip forward. I don't think this is much easier than actually playing the game, so I'm not disappointed, and I hope they enjoyed cheesing the puzzle.