Are your battles triggered by event, or are they random?
If they're triggered by an event, you can just add the number of enemies in the mob either before or after the Battle Processing call. If they player wins the battle, it means they've killed that many enemies. If the player loses the battle, it'll be game over and they'll have to load an earlier saved game, so the variable won't include the enemies in that mob.
Then, depending on what your event is/does, you can either have it conditioned by the variable being equal to or greater than the number required, or you can do a conditional branch after each battle & variable change, to see if it's the right value, and turn on a switch if it is. Then the switch can condition the event (this is useful if it's a common event and your mobs cover more than one map).