When you do the 'One Shot One Kill' level on veteran the end bit is difficult to survive until the helicopter comes and picks you up How i did this is when you first come out of the last building(where the abandoned amusement park is) you see the ferris wheel on your right and thats where you have to go to put Cpt MacMillan down instead turn around once you drop off the building and go down the path on your left The path is a dead-end but there's a door thats shut at the end on the right remember the way to get there Go kill the gaurds and drop off MacMillan now you will have 30 seconds until the gaurds start coming First run down to the white fence the goes through the middle of the area and place all your C4 around it then run back to MacMillan or to the tall buildings you can go between Once the enemies start coming shoot as many as you before they get close then run through the L shaped pathway between the tall buildings to get behind them and get to the doorway near where you came into the area Place all your claymores along the path down to the doorway(which is now open because thats where the enemies came from) warning do not plant your claymores to close to each other or they'll blow each other up Now simply sit in the doorway and snipe them when they come at you and when they shoot at you just go back into the doorway and let the claymores kill them(it helps to pick up a shot-gun when your going through the level in case all your claymores get blown up) Then once the helicopter lands detonate your C4 to clear out troops in your way grab MacMillan and jump on the chopper Hope this helps this is the way I beat this bit and it made it heaps easier because if i just sat behind the ferris wheel they would come up from all sides and overwhelm me

Just some things to know:

The reason you can't run straight to the door after you drop off Captain MacMillan is that if your standing near the door then it won't open and the enemies will come out another door across the map

Remember dogs! They've got a bad habit of getting past claymores and if your sitting in the doorway thinking your safe because you've got claymores between you and the enemy they can surprise you and kill you fast