[Partner to the End]
[Reach max compatibility with a weapon]
Weapon compatibility (the bar below the weapon's picture) goes up with kills. As it goes up, so does its attack. Get it to max on one character and you'll have your trophy. The best way to raise it is to have Gyuki at S proficiency on your team and, if possible, have the ?? (Harmony) attribute on your weapon. Then just go to any level such as chapter 3's ????? (Rescue at Yi Ling), and kill kill kill. With Gyuki and Harmony+10, you should max out after around 4000 kills. Of course, there's no need to really farm this. Just play with the character a lot and you will get it eventually.