Speedy Slayer Trophy

There's a pretty good chance you'll get this naturally as you progress through the game, especially when you get to the later levels that have a decent amount of enemies in each area. You should be doing your Speedrun on "Novice" which will allow you to 1-shot the small white box-like enemies. There are a lot of these little guys around the poisonous pools in Mirrorsea (near where Cyd is) so if you don't have the trophy by then, that'll be the best place to get it.