This is accomplished with a Slime Bomb, which you will find randomly throughout the game or which can be purchased at the shops you will find find after progressing the story.
Once you have a few bombs in your inventory, you can potentially obtain this at any time when 5 or more enemies are attacking you. In some of the "arena-battles" (where purple walls will block you in until all the enemies are defeated), mobs of Skeletons will attempt to swamp you, all rushing you at once. These are excellent opportunities to go for this achievement.
Personally, I forgot about this achievement until quite some way through the storyline. As luck would have it, I had some bombs in the inventory when:
You can also use this location to get:
: (I didn't have any standard bombs so I didn't try here)
Edit: Iceberg River is another good spot - check my solution for Boom!