To level up faster than usual, try using the same gun until all the marksman and elite challenges are completed. They provide huge boosts and will quickly level you up to unlock better guns. However, do not use those guns yet. Instead, keep on the challenge until it has completed. The third phase of the challenge usually ends with 1,000 experience points, which will bump you up a rank almost in itself. Also, do not forget about the odd challenges such as jumping 15 feet and surviving or falling 30 feet to your death. To get that one, if not previously unlocked, play the "Countdown" map and jump into one of the open missile silos. You will earn instant experience points for simply dying. Also, remember to go prone and crouch to get those challenges as well.
To level up, gain points, and complete challenges easily, find a TDM game of the map "Shipment", preferably with plenty of people. Also, make sure you have the Steady Aim/Martyrdom/Stopping Power perks on and select a gun you want to level up. Shipment is a very small close shouldertoshoulder map, allowing you to run around while holding down the trigger to take down as many people as possible. This is very fast paced and you should grab points very quickly.
Playing modes such as Sabotage and Ground War earn you lots of experience points because both games offer many kills. Sabotage has a long clock, giving you the opportunity to get many kills or challenges. Also, look for Sabotage on the Wetworks map as that map takes a lot of time to end.
Search for a Cage Match game until you become the host and no one else is in the room other than yourself. Then, invite a friend so he joins. Start the match and easily get challenges and rank up easily with your friend.
Use the following trick to level up and increase your KillDeath ratio. Cook and hold a grenade at camping teammates, then throw it at them. Quickly switch teams and the grenade will become an enemy grenade and kill them. You will not lose points for switching teams and can rack up huge kill streaks. You can also get all of the bonuses such as Airstrikes, UAV, and helicopter. Note: This also works when using an airstrike. Call the airstrike on your team, then switch teams to kill them.
Play Headquarters as soon as it is unlocked. In this game mode, for every five seconds your team holds the headquarters you get +5 experience points. If you are on the winning team (a team wins when they reach 250 points), you will get 250 experience points in addition to whatever amount of kills you get. If you are unskilled and cannot manage to get kills, you will still get points just for being on the team that holds the headquarters. This is a gamble though because your team might go the entire match without ever having held the headquarters, resulting in no experience at all.