You must create an actor or actors and make them belong to a gang "enemy 1" or "enemy 2". They must belong to the same gang. Then you must create an objective, choose character, make them belong to the same gang as the other actors and select the "Kill whole gang" option and turn it to on. Even if I kill the objective, the mission won't progress until you kill the whole gang. The actors must spawn before the objective. Hope this helps! Also figuring this out.