Are you up to the challenge?
When playing Call of Duty 4 online, you gain experience points based on feats you perform in the game. On top of your regular experience gained from killing enemy soldiers, you can also get bonuses for performing some specific actions. Many of these can be attained simply by playing the game, though some require special effort. If you set out to accomplish all of these Challenges, you will rise in level much faster.
The Elite Challenges are the final set of challenges that are given to you in the game. They come available a few at a time. Some of these challenges are nearly impossible to do, in my opinion. I've been at the max level of 55 for months now and still only completed half of these. You'd practically need someone on the other side to cooperate with you to accomplish these challenges.
Call of Duty 4 Elite Challenges:
The Brink (250 xp) - Get a three-kill streak when near-death.
This is pretty easy to get, especially if used with the Last Stand perk.
Collateral Damage (250 xp) - Kill two enemies with one round from a Sniper rifle.
The best way to get this one is find a doorway or higher traffic area and wait for two guys to come running through.
The Edge I, II and III (50, 100, and 250 xp) - Get the last kill in a match, 1, 5 and 15 times.
If you play long enough, you'll eventually get this one. It's tough to set out and do it.
Flawless (250 xp) - Get through an entire match without dying.
I honestly don't know how you could do this one unless you just hid the whole time.
Tango Down (250 xp) - Kill every member of the enemy team (four person minimum).
With a smaller group of opponents, this shouldn't be too hard.
Hard Landing (250 xp) - Kill an enemy that's in mid-air.
Catch some bunny hopper doing their thing and you'll complete this challenge.
Extreme Cruelty (250 xp) - Kill every member of the enemy team without dying (four person minimum).
I've never done this one, but it would be easier with a smaller group of opponents.
Fast Swap (250 xp) - Injure an enemy with your primary weapon, then finish him off with your pistol.
If you have the Last Stand option, this one should be easy.
Star Player (250 xp) - Finish the game with at least a 5:1 kill ratio.
Although quite difficult, a couple of good Airstrikes should help you with this one.
How The ? (250 xp) - Kill an enemy by shooting something explosive through a wall.
You'll have to use the 'noob tube' for this one, but it does work.
Dominoes (250 xp) - Kill an enemy by setting off a chain reaction of explosives.
This is a weird one because you may need help from others, but it can be done.
No Secrets (250 xp) - Call in UAV Recon two times in one match.
This one is very easy to get. By the time you reach a level high enough to complete this, you ought to be getting several UAV's per session.
Afterburner (250 xp) - Call in two Airstrikes in one match.
You'll get this one eventually just from playing.
Air Superiority (250 xp) - Call in two helicopters in one match.
If you can get two airstrikes, then you should be able to get the helicopters, too.
Fearless (250 xp) - Kill 10 enemies without dying.
Call in an airstrike or helicopter will help you get this one.
Counter MVP (250 xp) - Kill the number one player on the enemy team 10 times in a match.
If you get this one, it'll probably be one of those random things. It's hard to seek out certain enemies.
Invincible (250 xp) - Get five health regnerations from an enemy in a row without dying.
You'd have to be pretty lucky to get this one, but it's possible.
Survivalist (250 xp) - Survive for five consecutive minutes.
The only way I can think to get this one is to hide. The only problem is that there are very few good hiding places in Call of Duty 4.