In the battle event commands in the resource editor, you'll have to create a new enemy (or rather, duplicate one that you already have or something) and in it, at the end when you kill the enemy, make sure you include the event command Variable:[x] += 1. That means that everytime you kill that monster, 1 will be added to the cache in Variable X. Also, again, make sure your quest giver or receiver has the conditional branch making sure that it is checking that Variable:[x] >= n. N is how ever many of those enemies you need to kill.