alright. Best way then, would be to use a variable. When the NPC gives you the quest, set the variable to 0 (in case you've already killed monsters and it's been incremented).
Then in the Troop Events, right at the start of the battle, make it increment that variable by however many of those monsters are in that troop.
Then in your NPC event, just have it check if the variable is >= however many you're supposed to kill. It'll keep getting added to even after you finish the quest, so your NPC event should have pages for giving the quest, checking if the quest has been completed and giving you a reward if it has, and what to say after you've received the reward.