All Ghillied Up: End Of Mission:

after you set Macmillan on the ridge and have placed all of your c4 and claymores go into the shack/ticket booth that is in front of Macmillan and face entirely away from the window facing invading force and go prone and no matter how many grenades the enemy throws into the shack, you will not take any damage..however you will have to neutralize any enemies that see you and are going to shoot you