For this, you need to defeat 30 enemies with an 'Enhance Armament' 'Arts'.
I recommend charging your arts gauge against the Clamp Cluster in Cordea Plains (without defeating the clamps in the area. This will allow you to do infinite strikes against the clamp cluster to charge up your arts gauge.)
Use Kirito's Night Sky Blade to do this, or the Mirage Blade of Inferno (which you get at the end of the story). You can also do this with any other weapon that has the 'Enhance Armament' stat in the description of the weapon.
Anyway, once you've got a full arts gauge, you'll be able to either use it against the Crystal rock creatures the clamp cluster spawns, OR against the Thorn Bugs a slight way away from the cluster, which are normally around level 26. Kill 30 enemies with the Enhance Armament, and you'll get the achievement.