First make sure you have the “Evil Eye” Helmet as this doubles all lifeforce gained. Next head to the Everlight Facility and draw as many brown minions as you can as well as 5 blues (the blues aren’t required, but make gathering the lifeforce easier) and enter through the main gate, on your left is a 5 Minion switch. Activating this will release a constant supply of Gnomes.
Split the group of Browns into 15, 10 and 15. (if you have a full Horde size, if not scale down accordingly) Put the 15 on the right and left just above the stairs and blocking off the two "escape" routes either side and the 10 in the middle, all on guard markers (hit ) and use the 5 blues to gather the lifeforce (invisible when sweeping). Hardly any gnomes get through (if too many get by they stop coming and you must then kill the escapees in the square just behind you). If you put the minions too close to the entrance/exit of the Gnome cage then a lot of lifeforce get's trapped inside, hence the placement just past the stairs.
This will get you the minion Lover and Gnome Grinder achievements, as well as many gems for forging.