Successfully complete the game and save. Load the cleared saved game file to start just before fighting the final Boss. Instead of starting the battle again, exit and return to the cemetery. The fence that previously blocked your way will now be gone. Enter the area that it protected to find a bonus dungeon.
At the end of the Runes Of Oblivion there should be four Snake Portals. Defeat the Boss after these four Snake Portals to make returning to the map easier. Once the Boss has been defeated, go back in the entrance above the four Snake Portals then return back outside to them. These four portals will always be here. Each trip to them is worth 5,000 energy. Equip a high damage weapon with Haste to one-hit the portals for even easier energy farming. Also, after you have raised the Cursed Castle, the enemies on all maps become more difficult, and these portals will start dropping roughly twice their normal amount of energy (about 15,000 per trip). In one hour you should be able to get over one million energy. This makes energy farming extremely easy.