Once you reach the part of the level where you have to defuse the bomb on the door, have your team hold at the base of the stairs. Then, advance up the stairs and stand a few feet back from the double doors. Aim at the bottom of the doors and throw a grenade. Back away from the grenade fast so you do not die, and the grenade will go off. A delayed rumble will occur, and you will receive experience points. The amount of points depends on what difficulty you play at, and you will receive 2 A.C.E.S. Assault points for a kill with a grenade. You will fail the mission because it states that you "Disobeyed a Direct Order," but do not worry -- you will not lose any experience and can just reload the checkpoint at the bottom of the stairs again. Repeat the process several times. This will help you effectively unlock guns and rank up. Note: This was done under the Realistic difficulty setting.