In order to solve this quest, you only need at least 6 trained troops. once you hit 6, kick the others out the party. As soon as you that, you should get a prompt that asks you if you want to send the trained troops back, just say yes. The easiest way to train those troops is autoresolve, or if you like fighting your battles, assign these troops to a specific number (Party page key P, the roman number next to the troop, default 1) and send them on alone to fight looters while you loiter in the back, if there are a bit too many send some else to assist.
Should you fail this quest, don't toss the trained troop away, because you can keep them for another quest of the same kind in another village.