Thank you for playing my game! I agree that the skull collection thing could've been explained at all, I will improve that feature after the jam.
But just for you to know, the skull collection becomes available only when the "boss" (the larger enemy) dies. Then you have 30 seconds to collect 8 skulls in order to open the hatch (witch leads to a win). As soon as the boss dies, the doors at the top start spamming enemies. If the boss respawns (30 seconds) and you haven't won yet, you drop all the skulls, and have to start over (kill the boss > collect another 8 skulls > find the hatch to escape).
The click-to-shoot thing is a recurrent complain, I will address that issue after the jam (by having a rate of fire and just holding the lmb).
I'm glad to see that you liked the sfx xP
Thank you very much for your feedback!