Kill a Thousand Gnomes - Overlord 2
This quest is unlocked just shortly after the start of the game, but it can be easily completed after the Green Hive has been rescued from the Everlight Facility. Return to the Everlight Facility as the Overlord via the Throne interface or a Netherworld Gate, and use the Brown Minion Gate there to call forth as many Browns as you can.
Head through the front gate, and down the steps. Walk across the grassy patch, and turn left at the top of the steps. Sending five Minions onto the pressure switch will open the Gnome cage; Gnomes will continuously pour out of this cage every few seconds, provided that at least some of the Gnomes in the last batch that came through have been killed.
Open this cage in the Everlight Facility to find a steady supply of Gnomes.
If you stand at the bottom of the steps that lead up to the cage and keep the RT/R2 button held down, you can continuously send your Minions in to both kill the Gnomes and pick up the treasure they drop. Consequently, this is a great place to harvest Brown, Red, and Green Lifeforce Orbs, as well as Fairy Gems. Make your time spent here even more effective by having the Evil Eye helmet equipped, which has the effect of doubling the worth of picked up Lifeforce Orbs.