This is a fast way of taking care of most of the Bosses when you are not in your Gears. You will need Billy and Rico besides Fei. When you start the battle, use Billy to cast Goddess Call on Rico, then on the next turn use it on Fei. Once these two are "hasted", use Billy for healing purposes, Do not attack with him as Rico and Fei will easily be able to finish most Bosses without Billy wasting his ammunition. Have accessories that boost attack power on the both of them, as well as use their respective power up skills. During the Boss battle, do one weak attack then cancel the rest to get as much AP as possible. Once you build up to 28 AP, go to the combo menu and deal it to the bottom combos, adding the left over AP to weaker ones. Fei's combos will do about 2000 damage if powered up correctly. Rico's can do close to 2400. The crucial thing is to use Goddess Call, as Rico is an extremely slow character.