the way to kill enemies is by attack dashing(x, l) in the direction of the sort of bubble around the enemies so if the arrow came from the bottom left u would need to input a directional attack dash diagnoal up right (keys: up, right).
you are right though, we didn't have enough time to make a proper tutorial, but the killing does work as intended, it's just a little unclear.
Hope you try it again with this info and play it to the end ^-^