Got around to playing this finally, made it to wave 20. Once the timer started to really matter and the sonar ability was limited the gameplay loop felt engaging. Would play again if multiple levels were featured and some variety in the music.
Don't see how the girl turning into the bat is really relevant to the game, if I were to suggest something maybe a human form mode where your meter drains faster but you can directly run on the ground making it easier to touch people.