The 4th challenge might be the easiest one of them all. Even I can do it. Choose a gun you normally never get kills with at all, for me the plasma pistol or sniper rifle, and in firefight only use those guns, grenades, melee, turret and vechiles in firefight mode. Place what weapons u never use and the stats from ur match. A solo match with 3 skulls on(except no hud) and can be on any difficulty u desire.
Happy hunting hopefully