Overlord

For this achievement, you don't have to have accumulated 10,000 lifeforce at any one time, you just have to have harvested 10,000 lifeforce over the course of the game. So feel free to create weapons and armor as you see fit, it won't hinder your ability to get this achievement.

Nonetheless, this will take a lot of grinding to get. The easiest way is probably fighting Battler Beetles in the Dungeon. I was getting about 1000 lifeforce every 15 minutes that way, so defending on how much lifeforce you harvested throughout the game, you'll probably have to grind for 1.5 to 2 hours in the Dungeon for this achievement.

*Tip from Ater Magus: If you have the Abyss DLC, equip the Harvest Helmet, which doubles the life force collected. This means you can halve your time grinding in the dungeon!

After Reading a FAQ on Overlord I read a VERY easy way to get the lifeforce (plus a bunch of cash) while not even playing.

I found it from this faq : http://www.gamefaqs.com/console/xbox360/file/932853/49053

this is the method used:
"-Emailed to me by Jeff: i found out a much easier way [to get money]...go to
heavens peak city and kill the peasents near the white robed men ( clerics)
they will revive them and they will keep dropping money. use a marker to put
ur minions near or on the helpless peasent and the will constantly attack him.
stand over top of his body and u will collect loads of cash and souls for
upgrades on weapons and armor (i thinku can only do it after u beat the game...
its near a gate....its one of the sections where u had to face kahn and there
were sum villagers with white robbed men...)"

I was able to send 1 brown minion on the right of the stairs that are on your left when you spawn. I than set the controller (plug n play to keep from disableing) down and watched tv and played on my other 360 while waiting to get a boatload of cash and lifeforce without doing much of anything.

Not trying to rip off "Jeff" or the creator of the FAQ I found this in, but an attempt to help lazy others like me :D

*Side note: When I did this I also left it run all night while I slept, because of the citizen spawning with a mug everytime it made things very laggy with 1000s of mugs on the ground. If this happens just leave your minion there and go through the portal to the tower, otherwise it might freeze up.