Great game! Very cute graphics! Gameplay has lot of variation and the different enemies work good together, but I would suggest to split it over time rather than having all attacking at once. You could start with 2 enemies and by varying which enemy/boulder appears for a given 'level', and the attack speed, make a really long and challenging gameplay! It also deserves more detailed backgrounds IMO, hand-drawn rock-walls and so on!
One thing, and it should really be the first rule of all game jams: using Z and Y as controls is a poor choice because many languages have them switched on the keyboard, so imagine me playing: x .. right move, y ... left move!! But I did solid!