^ the trick to that is to use all your claymores in the center area where all the grass is (the little field next to the building with the pool in it. Set each of them so they are behind something and the enemies wont see it. There are some abandoned cars and walls and bushes that are perfect for hiding them next to. Be sure to put them so that the red lasers are acutally sticking out from behind them. If they are completely behind the obstacle, they wont do anything when tripped. For example, if youre hiding it behind a car, put it near the rear bumper so that the red lasers stick out a tiny bit from behind the car. You dont want them all the way out from behind the car, just like half the laser behind, half the laser sticking out. Remember with the claymores, the kill zone is between the red lasers, so set those up in a position so that the blast area doesnt have any obstructions. For example, if you are hiding it behind the car, make it so that the laser closest to the car is parallel to the car, and the other laser will be sticking out at an almost 45 degree angle from the car. When they trip, they take a second before they blow. So if an enemy walks by it, it will trip, they wil keep walking for another second and by the time it blows, they will be directly in the blast area. This is also a good strategy when playing online. If you set a claymore by a door, place it about 5 feet back from the door, on the same wall that the door is on, and make it so the laser closest to the wall is parallel to the wall. When an enemy walks into that room, through that door, there is no way they will be able to avoid it.
Also, are you sniping when waiting for the helicopter, or did you pick up an AK and use that? If you didnt, ditch your pistol and grab an AK from a dead bad guy when you can. I found it easier to snipe at first. 90% of the enemies are going to come through the field where you placed the claymores, so the best thing to do is get up on the ferris wheel and get into the cart at the very bottom. Crouch down, and from there, snipe the bad guys before they even have a chance to trip your claymores. If any get close, whip out the AK and put three rounds in them. Youre gonna have to conserve your amma, so when you shoot them, Tap the shoot button to fire a single shit, but do it three times. If you go full auto on them, youre going to waste so much ammo.
As for placing Cpt. McMillan, if you are standing on the ferris wheel looking towards the field where the enemies are gonna come from, place him to the far left of the ferris wheel. Once you set him down, stand behind him and check to see that his field of vision is unobstructed. If there is a building in the way, or any other obstacles in the way, adjust his position so that he hast he biggest field of view possible. If you do this right, he will take out 50% of the enemies for you.
Just try to kill the enemies before they trip the claymores. If you manage to do that, the claymores will serve a better purpose by lasting longer and now blowing right away. If the enemies manage to make it past the claymores, and you are still on the ferris wheel, they will go the bumper cars, so remember to check over in that area frequently to see if any made it by.
Sorry this is so long, I really get in tot his game. GOod luck.