Nice game and cool music (I can feel the VS inspiration from it), although as already mentioned it does not following the beat. As a rhythm gamer that actually trips me up lol
Would be cool to maybe have it synced and with increasing difficulty allowing for two or more arrows on the same "beat". But I also know how hard it can be to make it match, even more so when it runs in a browser but it doesn't seem like the perfect window is too tight.
Since you have enemies coming from the sides why not use those as arrows (similar to Undertale's Undine fight) and place the arrow receptors in the same manner as it is on an actual pad and you have to press the correct arrow when the enemies is on top of it? That would give it more of an unique feel.