Killer Prime and Gambler are one of the more common PvE farming recommendations. KP removes buffs and debuffs the enemy, and Gambler deals extra damage the more debuffs the enemy has.

There are so many options these days, and it's always worth making sure you have a few FC on hand in case you need to spend them during events. My general advice to everyone would be Killer Prime, but I'd also recommend saving your FC until you need a character for an event or one of the frontier-dungeon achievements.