This was really polished! I especially liked how the option to choose the next challenge flowed right from the rewards of the previous challenge. I had a lot of fun with the different encounters and I don't think I saw the same one twice. I tried the hardest challenge most times for fun but I think you might need something like a time limit that forces the player to make tougher decisions so they can't choose easy every time. Also, I like how the player wraps around the screen but one problem I saw was it makes it easy to trap enemies in the center of the screen by just moving back and forth on the edge. Still though, this was really fun!