This is GREAT. The level of polish throughout is very impressive, the intro animation was very well-animated and a welcome surprise, and the gameplay is fun and fast without being confusing.
I haven't gotten to the boss, though, because I can't seem to get an A rank on level 2. Maybe the requirement should be to get a B rank? I got tired of trying over and over. It would also be helpful to know how many points constitutes each rank, like in Sonic games.
I also find it difficult to successfully target enemies, because the range for the "homing attack" (X) is a bit short, so sometimes I've moved out of range by the time I can react. Other times, the homing attack time limit runs out right as I'm pressing X, so a more noticeable visual indicator of its time limit would be helpful. It's also nearly impossible to hit enemies that are lodged in little craters of land, like in level 2.
Despite these issues, I still enjoyed the game. Keep up the good work. I'd really like to see more of this!