I found the best place for this achievement to be in the Underground River just before you enter the Ancient Factory.
From the final save point in the area you can go left or right, choose the path to the right, if you run into Toripo you went the wrong way, just go back the other way.
Once you deactivate the blue barrier there are many many Kelolon in this area that make it easy to get the achievement.
Walk into the middle of them, and activate your encounter circle, just keep turning it off and on and checking the number of enemies that are selected. If you get attacked just flee the fight and try again. Each time you flee all of the enemies around get closer, only took 3 tries to get 10 enemies in a circle. Once you have 10 or more enemies in your circle, kill them all to unlock the achievement. The video shows it all(Sorry about the low quality!)