Attempt this technique towards the end of the eighth mission if you’re in need of essence. At the last save point (before Rachael runs into the creepy guy), you will find a door which leads to an alleyway. This alley is a dead end, but enemies will spawn and constantly respawn on both sides of the door in the alley. If you run back and forth here, you can get endless amounts of essence, as the enemies, and the essence they carry, will never stop respawning as you go back and forth through the doorway.