I found you can actually change the difficulty with the up and down keys. So W and S for keyboard, and the up/down buttons on the D-pad. There's Beginner, Easy, Medium and Hard. I haven't tried the other ones myself yet, though. So no idea if they actually work, or how hard they are. Edit; The other modes sure work. Hard is as unbeatable as you'd imagine it to be. Easy is about the furthest I can reliably go.