Nice game, but the controls are just bad. When you hold mouse button and the hero hits an enemy, the hero bounces back, moving away from the mouse and therefore accelerating. That makes it pretty much impossible to retreat at higher speeds unless you manage to release the mouse button in time(which, with faster speeds, is hard). Either make the location of the hero static relative to the screen or make the movement speed depend on the location of the mouse relative to the screen instead of to the hero.