Idea is unusual, required my to die three time to understand, what i need to do, maybe some text with objectives could help.
Overall, controls are not good, using mouse is comfortable, but clicking it a lot isn't nice, maybe holding mouse button to move is better? In spite of that, gameplay is competing, but my records was always 20 second, despite of trees count, it looked, like they were not helping much. So, it made me quit, as i didn't seem any goal. Graphics is simple, and i liked it, but maybe fireballs could be more red or orange, so you understand that it is "fireball".
But, great idea, would look on your other projects!