Only the mobs in the darkness areas/ transition areas count as part of the strike, anything in open public areas such as the mission start points don't count as part of the strike. Sitting there killing endlessly respawning level 8s is the waste of time, you might as well be doing patrol. If you're doing a lvl 24 strike, only the lvl 24 enemies you are fighting count. If you kill a lvl 8 and get a blue, good for you, but it will be a level 8 blue drop not a 24.