Really interesting idea to degrade the ground where enemies are killed. It adds an interesting layer of strategy over the game when deciding where to kill enemies. It might be cool to add bonuses to the greenest "purified" grass that would make the player want to stand there but using the power would just lead to the surrounding area becoming more cursed and the powers going away.
Best time I got was a 2:41 with 45 kills. The main character sprite is nice too - they spin very well.