Overlord II

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.

When you find the green hive in the guarded empire building and have to transport it back to your tower you will find creatures that shoot blue lightning (name?) there are a couple that you need to distract in order to kill them more easily. In order to do so you there are 2 buttons in that area you can push. Pushing these will open a cell and gnomes will come out. The stream of gnomes is endless, so you can kill all the gnomes you like to get extra lifeforce and this achievement.

I'm not 100% sure if this works for both buttons, but for the second one you find it worked for me on 2 different occassions.

An alternative to the above guides can be done during the main storyline, while powering up the dungeon heart in the wasteland sanctuary.

After you have charged the stone at the first shrine you will drop and gnomes will steal the heart. Your task is 'retrieve the heart from the gnomes'.

When the heart is removed an infinite supply of exploding gnomes arrives. All you need to do is stand in the water and place all your minions in front of you on a marker. The exploding gnomes cannot reach you and your minions will kill them over and over again.

in the level when you are trying to charge the tower heart theres an area where exploding gnomes come out of a hole in a clearing. theres a tapered path that leads back up towards the netherworld gate. . place all your minions on this tapered path and stand behind them. This way the gnomes will just keep running towards your minions and they will kill them.

The best place to kills Gnomes is in the Everlight Facility, after you open the main gate and enter with the Overlord you will see a prison full of gnomes to your left, just open and hold towards the opening that your minions will kill them and capture Lifeforce Orbs, this achievement is part of a side quest that you get at the beginning of the game, so you will also see your progress in killing a gnome, you can also unlock the "Minion Lover" achievement by doing this method.