Play Episode 1: Mission 1 - "Hollywood Holocaust" on the easiest difficulty. From the start of the mission, fall down the ventilation shaft, and run directly to the alley behind the movie theater. Use the pig cops as platforms to jump onto the small lip running along the wall. From there, go to the bridge. Once on the bridge, run past the pig cop and end the level to get the "Alien Maggots Are Gonna Pay" achievement.