To get to the rank of Elite, you need 400,000 experience. Your experience gained is combined throughout single player campaign, terrorist hunt and multiplayer adversarial. Go to Rewards in the Main Menu to check your progress.

Here are some of the best methods to rank up:

Method 1

Find a bunch of people willing to boost.
Ideally, you will want to have enough for 5 on 5 to get the maximum possible exp... any more people will still net you the same exp.
Have someone host a Team Leader match (player, not ranked) on Kill House with enough private slots to prevent other people from joining.
In this game mode, with 10 players in the match, you will get 200 exp if your team kills the opposing leader, and an extra 200 exp if your team wins.
Therefore, you will want to switch between the team that is winning so everyone gets an equal amount of exp.
I suggest giving each team ten turns before you switch, to avoid confusion. Just make sure you're keeping track.
Both teams go to the roof or long catwalk and the leader of the winning team must die first. The leader of the losing team can see who he is, so have him kill him.
Once he's dead, the winning team then kills the entire other team.
This means the losing team still gets 200 exp, while the winning team gets 400.
After 20 matches which should take about 10 minutes if you're quick, everybody in the game will have received about 6000 exp.
That is the equivalent of killing 60 terrorists per minute on realistic difficulty... an impossible feat.

Method 2

Get your ACES levels up.
Every other level in ACES gives you a very large experience boost so it's highly recommended you max out all of your ACES levels.

Method 3

There is a story mode glitch that allows infinite respawns at a certain point in the story. See this [url=http://www.youtube.com/watchv=8AciKwatRpM]youtube video[/url] for how to do it. If you'd like to get this while you sleep, see this [url=http://www.youtube.com/watchv=mEYRmqIw4WQ]youtube video[/url] that requires you to have a wired controller (or play/charge kit) and tape the left trigger down to stay in cover. This way, the turret guy will not kill your teammates, but your teammates will kill them and you will get +5 for every time they kill the turret gunner. This is slower, but since you're sleeping while it is happening it is better than nothing.