Really clever idea. I like how it takes a while to regenerate letters so you're forced to get creative. There was some guess work with letters not hitting the cells I expected, but I'm not sure how you would fix that.
The graphics and the audio are charming. The duck's expressions are a nice touch.
I wish the score counted the enemies killed rather than the rounds passed. Maybe some score bonuses if you kill a bunch of enemies with one letter. I could never seem to get further than round 2 so it would be nice to have some more granularity.