I've been in a struggle for a couple a days trying to figure out how to make a challenge mod.
Allow me to explain, If you get a certain amount of kills say 50, 75, 150 and so on or if you get to a certain level (I'm only saying this because I'm going to put it in one of my mods that has my level mod), it unlocks the challenge saying that you got that many kills or got to that certain level and it gives you a reward and it saves so you don't have to unlock it over and over again.