Star Ocean: Till The End Of Time

Not to be confused with the Status Skills you level up with Status Points, every character's Battle Skills and Symbology Spells have mastery levels attached to it that increase as you use them. You can view the level of a Battle Skill by going to "Skill Setup" from the main menu and looking next to CP. Unfortunately it's a bit harder to see how Battle Skill's level, as they go from "1 to 5000" instead of "1 to 10" like Status Skills do. You shouldn't have to worry about this trophy though, so long as you give one of your AI partners a battle skill and leave it equipped, you should have this trophy by the end of your first playthrough. (For reference, I never removed Cliff's "Sphere of Might" Battle Skill when he first joined the party and it reached mastery level 10 at 1500 points)