The game went from really hard to really easy once I figured out that you can't be hit when backflipping and enemies can't shoot you if you're standing in the same place. I didn't find the game fun at all, just tedious because of the hight number of kills required for the doors to open.
Addictive and a lot of fun! The accuracy acted kinda strange at one point. One level told me I had -153% then the next said I had 2000%. Seemed to fix itself after a level or two though.