This solution will get you both the "Minion Lover" achievement for collecting 5,000 lifeforce, and the secret "Gnome Grinder" achievement for killing 1,000 gnomes.
First you need to forge "The Evil Eye" helmet; this will double all lifeforce you pick up. (It is also the helmet needed to go with the Elemental Armor for the Blue Steel Look achievement.)
Next, go to Everlight Facility and draw out all brown minions - as many as you can. I recommend at least 40.
Go out to the main road and hang a right toward the facility. Go down the long hallway to the first room. In the very first room you enter, there will be a floor switch on the left side in a short hallway. Plant all of your minions about 10-15 feet from the doorway where the gnomes will come from, then draw back about 5-8 minions to keep with you.
Sweep your minions over the floor switch to release the infinite supply of gnomes and let the carnage begin. Just run in occasionally to pick up lifeforce and use the minions you have with you to chase down any that escape.
Just watch out for the gnomes that attach to you and explode. If you get low on health, just run back to the Netherworld gate and heal up. If the gnomes stop coming out (happens occasionally) just leave the map and come back.