Heh heh, I managed to break it! When I was in the battle mode, I had my second character attack an enemy that was still being attacked by my first one. The first one killed the enemy, and then it erred out, probably because the target no longer existed :P

I think that it could be a good idea to have health--sometimes you are moving when you get killed and the shield deactivates immediately. Also, it is possible to shoot a jelly thingie and have it respawn on top of you. Maybe you could have a short delay between the jelly respawns, or make it so they cannot actually spawn too close to the player.

Seems to me that the dropping of crates is a bit fast. Maybe there should be a delay between drops if the down key is accidentally held.

But still, even with the chords you have, you could do it. Get chord lists from songs, and then have the song playing in the background. Have the chord the player is supposed to be playing be shown, and give points for playing the chord correctly!