On MW2 online, it doesn't take much skill to kill a player in any game mode, (especially in hardcore, when each player has around 30 health points, depending if a player has painkiller or last stand on). These players might run around and knife others with ninja pro or commando, play very defensively, stay at only one position (most likely not, because these players are usually exposed on the mini radar and other players will know where they are), play with a stealth class or staying defensive and offensive at the same time moving and switching positions after a kill, (most ideal for impressive gameplay and score).
Killstreak rewards such as UAV, Counter-UAV, and EMP from your teammates will support you if you're trying to get an impressive score with reward setups such as Harrier Strike/ Attack Helicopter, Pavelow/ Stealth Bomber, and the AC130/Chopper Gunner, or even an attempt for a Nuclear Strike. If you're not confident that you can achieve the 7th killstreak award without Hardline or support from your team, start with UAV or Predator Missile as you're first reward if you want to get a good score. You'll eventually get better with every match and practice.
If you're starting with UAV, try these: [UAV, Predator, Harrier]; [UAV, Harrier, Pavelow]; [UAV, Harrier, Chopper Gunner/AC130]. These setups will also help your team because of the UAV.
Up above, are easy setups to get, but if you want a challenge, try these: [Predator, Harrier, Chopper Gunner/AC130]; [Predator, Harrier, Pavelow]; [Harrier, Pavelow, Chopper Gunner/AC130]; [Harrier, Chopper Gunner/AC130, Tactical Nuke]. These setups will be more difficult to get if your team does not support you UAV.
Don't frustrate yourself by attempting to get good scores with the setups above in every match. If your team does not support you with UAV, try to get good scores by getting UAV by yourself. By the way, the alternative rewards such as 7th killstreak Helicopter, 9th killstreak Stealth Bomber, and 5 killstreak Sentry Guns aren't as efficient as the other alternatives for the same kill.
In certain maps, air support can devastate the enemy team. However in some maps, air support is almost useless. In wide open maps such as Wasteland, Afghan, Rundown, and Derail, killstreak rewards such as Harrier, Pavelow, AC130, and Chopper Gunner can devastate the enemy team and can even help you get a Tactical Nuke. In maps with a lot of buildings such as Skidrow, Karachi, and Sub Base, the high end air support won't be as effective. Just be smart and choose the setup for the right situation.
Players also use perks to their advantage. Perks like Sleight of Hand Pro, Scavenger Pro, Stopping Power Pro, Hardline Pro, Cold Blooded Pro, Ninja Pro, Commando Pro, and Steady Aim Pro are one of the most common and recommended perks to make combinations with.
One last tip, always mind your connection bars in each match. If you get killed easily by another player, check if they have faster connections then you by checking the bars. If you have one, two, or three bars, don't expect to kill others easily who have 4 bars.