I have seen many games that just randomize input but they tend to be more annoying than fun. Yours is fun, because the arrow changing all the time makes you move constantly. You want to be close to the enemy because that allows you to be on their right side quickly in order to shoot them, but that's also risky because you have less reaction time if they shoot. Really good implementation of such a simple concept ;)
Also, pretty minimalistic but completely functional, snappy and polished. Loved the particle effects to signal destroyed enemies, good job!