Conan

You can use the Fire Rain armor power or Void power during the last level there's a point where you have to push a pillar over to create a bridge, it begins to crumble as you cross it. When you get across, you'll fight several Oilmen, just wait until there are enough enemies around. All 10 must die at exactly the same time or it won't unlock. Like in the video below.

Mine popped on the Last boss. When you go to turn the third wheel several Oilmen will spawn. You should use the Void armor power to take care of them quickly, making the final fight easier. It may take a couple of tries.

Credit to RedScorpionBR for the video.

I did this and it totally works! (medium difficulty)
I just got this on the Dark Prison level. When you land in this level immediately run around the circle and wake up the ghosts. Oilmen will be spawning from the black pools during your running around. Head to the left of the screen where there is another pool and treasure chest. This wakes up more ghosts and spawns more oilmen. Run back to the original circle and draw as many ghosts and oilmen to you as possible. Roll/Run back to the left area where the chest is located, but stop the the choke point between the two areas. What you are doing is holding out long enough to group 10+ ghosts and oilmen together. Drop your fireball and pick up your easy 40 points for the achievement. I did this on the easy difficulty and it took me all of 2 minutes to figure out. Enjoy!
All credit for this goes to Ralph The Rogue from x360a.org

During the boss fight with the serpent queen, she will summon oilmen throughout the battle. On the second or third wave, she will summon exactly ten. Get them grouped together by running around and making them chase you, and then use the Fire Rain spell.

This is the easiest method, but it's still tricky and - unless you luck out - it will take a few tries to get the timing perfect.