Scar with silencer, trying to get him kills or get him killed? If you're a high enough level, use the ACR with silencer.
The trick to getting a high killstreak is to learn every map inside and out, all the turns, good sniping/camping spots, for instance, Highrise, on top of the building, on the second story of the other building, or on the crane just outside the map, all are easily accessible IF you know how to get there.
This is important, however, you must use a silenced weapon, otherwise you'll compromise your position, and, after you get 5 or so kills, move from one spot to another, EXERCISING EXTREME CAUTION! You don't want to get 24 kills then get sniped by someone while moving between spots. Remember, Flash/Stun grenades are your friends.
If you don't move between spots, one of 2 things may happen:
a) The enemies may start avoiding your spot, ruining your chances of getting a nuke, or,
b) The entire enemy team will stop paying attention to your comrades and focus on you, ending in you being gang raped by 8 players at once tearing through your body from all possible directions.
The thing I do most, is use a hiding spot quite often, then, when the coast is clear, move out of my spot, while keeping that spot in my line of sight, so when the enemies come to gang bang me, I can pick them off from a safe distance without them knowing my location. I have got over 30 AC130's and over 60 Chopper Gunners (I dont use nuke because the odds of me actually getting it are low, since my highest killstreak is 24 and I only got that once) using this method.