Crew Levels are powered up by the experience you'll gather in combat. Through defeating enemies with special abilities, capturing bases, rescuing allies, and by using the Brothers' Sake Cup skill, you will gradually fill up their bars and watch them level from 1 to 5. While the skill sheets themselves offer a BONUS level as well, for this trophy it is not necessary. I would still aim for some of them, in order tog et some of the Rare Silver medals that you'll need to character maps later on.