Once you reach Disc 4 (When you get the Invincible Airship) go back to the city of Treno. Near the save mog there is an item shop that has a monster under the grated floor. This monster will change periodically but once you reach this point in the game it becomes the Behemoth. Ask the clerk about the monster and tell her you wish to fight it. Choose Dagger from the list of characters, turn on the max damage cheat, or if you have the Odin Eidolon you can use that to easily defeat it. Below is a video guide for those of us who are visual learners.
*Updated, This achievement is not missable. credit and thanks to Lavindathar