I've made a quest objective where you have to kill all enemies in a room, then the quest will proceed to the next stage. But I only know how to do that with 1 enemy, not with multiple. So how do I set a stage when multiple enemies are killed?

One way would be to have each death advance to a stage in-between the two stages you are talking about. That intermediate stage then checks if all the enemies, which I assume are in ReferenceAliases, are dead. If all enemies are dead, then the quest is advanced to the next stage. If not, then the quest goes back to the previous stage and waits for the next attempt. There may be more elegant solutions, so you might want to wait a while in case a better method is posted.