You dont need to lock lobbies. All you need is 4 people with 2 controllers, so you have 4 people in your party (you and you 2nd controller, your friend with his second controller) then you have the other 2 friends with their controllers turned on waiting to join your session (team tacticle) then you just do headshots......do it in whatever game mode you get, doesnt matter. You also must have tacticle insertion unlocked