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.
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the level of the AI you kill does not determine the level of the item the engram will drop. otherwise there would have been no point of farming the "treasure cave"
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A blue is a blue and they all have a chance to turn into something more. Otherwise noone would of gotten anything from the Loot Cave farming. And noone said to sit there killing the Level 8's as they respawn either.