Okay so, its actually extremely easy and uncomplicated.
for this method, you will need a controller, even if you don't use one.
connect controller, select single building 21, not 3 man squad. you will instantly get a game with nothing but bots, so no worries about pvp.
then just go straight for the 3rd floor, do the mission in the lab, blow safe and grab the crate, jump out windows, go to basement, wait for the gate to open. Just like that.