Clear every stage of hostiles to power up the exit pad, then step on it. In Endless each stage is a new, larger map with tougher demons. Find the plasma rifle from stage 2, and watch out for burning skulls that lunge at you. Soldiers drop shells.

In Possession the trained agent plays the marine and you take over its demons, one at a time. It only knows where you are when one of its sensor rays (the red fan) touches you, so stay out of them and ambush it. When your demon dies, pick another. Stop the marine before it escapes.