I think I figured it out, you have to wait until the enemy is within the Yin-Yang watermark thing, and then click the direction of the enemy you want to slash through. Then the trick is not falling off the map.
I probably should have tried mashing the spacebar. Anyway, nice game. I would have increased the spawn rate as time went on until there is a whole rain of ninjas from the sky, but I only made it to 22 so maybe you did do that and I'm just not good enough to see it.
