Blades of Time

Alternative location to obtaining this achievement:

I obtained this achievement after getting 'earthquake', when you go to the practice arena and have to destroy 10 enemies. I executed the earthquake when 5 enemies were close together and the achievement popped.

This is possible in multiple places, the best place during the story is very late in the game when you get something that makes you much more powerful (NO SPOILARZ). You'll be attacked by a ton of enemies and this is very easy to obtain, thanks to your increased attack power. Just use a Massive something skill (ignition, heavy strike, etc) when a bunch of enemies are around you, and you might even get 10 enemies at once as well.

If you don't want to replay a stage, this is also possible in outbreak mode. Each time a group of enemy units spawn, there is 6. You just need to level up enough that you can get a Massive something skill and then use it.