This trophy is titled "Devil's Advocate" and it's for using Ragna's "Gauntlet Hades" 100 times. This can be done across all modes and is a relatively simple achievement. Instead of sweating the input, simply flick right on the right thumb stick and you should hear Ragna yell "Gauntlet Hades" as he lunges at his foe. Only flick the stick once to do the initial lunge and repeat the process and you should have your trophy in minutes.