1) Kill 50 zombies with the Olympia. You will hear a completion noise once you are done.

2) A piece of Chalk will spawn in one of 3 places around the map that you will need to pick up. It could spawn:

-On the corner of the collapsed ledge you can fall off of, opposite the Candy bowl.
-On the table left of PHD Flopper.
-On the table opposite the corpse to the right of PHD Flopper.

3) Go to the right of the zombie barrier, the one on the right of PHD Flopper, and you will be able to draw the Ray Gun wallbuy. NOTE: You cannot pickup the Ray Gun if you already have it or the Mark 2.