After you beat the game, a new location will be unlocked on your map. Head to this location, and enter. In the first room, head straight to the stairs. The stairs are littered with cannons and monster generators. When destroyed, they will give 15,000-19,000 energy per kill. Go up the stairs into the next area, and then back down. This will make the enemies respawn. You can repeat this process for as much energy as you'd like.