Nice concept! Has good controls, etc. At first, though, I didn't know who was the player and the enemy, so I started clicking the mouse randomly and, upon seeing nothing happening, was about to give a low rating. Luckily, I thought of giving it another try and could see what's happening!
Just a suggestion, some SFX could've been good!
A small problem is design, though. Staying in a corner would help you score easily, could do something about that, like making the arena a circle, etc...