This trick requires plenty of skill and practice. First, create the following classes:
Primary - gun of your choice, and have the grenade launcher attachment.
Secondary - One-man Army.
Equipment - Claymore.
Special grenade - Your choice (Stun recommended).
Perk 1 - One-man Army.
Perk 2 - Danger Close.
Perk 3 - Your choice.
The same as Class 1, but with C4.
Note: If you are trying to get a nuke, it is recommended that you use the kill-streaks Harrier strike, AC-130 (or chopper gunner) and Tactical Nuke.
Afterward, join a match (Domination recommended) with your Class 1 selected. Find a room with one entrance close to a flag. As soon as you enter, hide a claymore by the door or ladder. After that, switch to your Class 2 and plant C4 on the claymore. Then, use the grenade launcher to get kills out the windows while taking cover if some tries to hit you with a grenade or rocket launcher. If you run out of grenades for your launcher, use One-man Army to switch to Class 1 and get more. If notice that you get a kill from your claymore and C4, replant them. Very rarely will you only get a hit marker because of Danger Close and you are using two explosives.