Now as the earliest place to get this is as part of the story, spoilers shall be warned of!
If you don't do it at this point then when the arena is open proper (after killing the crocodile) you can fight any of the boss battles, and finish them off with a well timed
overpower. The moment their health reaches zero because of the attack, the achievement should pop.
You can obviously do this whilst also trying to get
Let's keep cheevin' together!
EDIT: I have had numerous comments of this not popping for people. I think this achievement might bug out for a few people and for that I apologize but as of yet have no solution.