I have 4 of the "cerebellum pieces", painkillers, and two game cartridges. I think the last thing I did was beat the movie theater boss. I've been running around in circles and not sure what I'm supposed to do now. Even a hint would be good.
I couldn't find the cave on the beach using Arthur all around there (do you mean the beach with the 4 doors?). I did find a strange anomoly that I'm not sure is a bug or a feature though:
Past the date puzzle before the boss where you get the rubber mask there is only one door in the full version. In the Youtube videos of the demo there is two doors and the right door leads to a room with a slide and an enemy that activates with EMPATHY. I've randomly stumbled into that room walking around the trees between the high school and the cardboard alien cutouts.
Ah sorry I should have been more specific. If you go directly down from the Chitin MA sign you should find it. I've added a gif below to better show what I mean. The room you entered is a secret room actually! I've moved it to a different location from the demo. I'm planning on adding a lot of secrets like that in the near future, they will all have to do will ARG elements I have planned
here is the gif:
