A partner is required for this trick. Create an online team deathmatch game and set friendly fire on reflected damage. Then start the game and have both players go into the room all the way in the southwest corner (the one with a furnace and some boxes in the corner). Have one player jump on the very top of the boxes while the other player stands in the same room but not close to the other person. Have the player standing on the boxes throw a grenade at the other person and try to jump at the same time the grenade goes off. That player should try to jump onto the thin piece off wood above the window to their right. If successful, jump from the window to the wooden planks on the roof. Then, carefully walk over the planks and around the edges until you get about directly above where you started. You can then jump to the outside of the level.