Want to play split-screen with friends, but hate the levelling up? I can certainly understand, so do this:
You need at least 2 controllers, 3 is better. The max enemies in split-screen is 2, so a 4th controller won't really help unless you desire to boost 2 players/shoot 2 players.
Set game mode to Search and Destroy, unlimited time/unlimited lives/1 point to win (plant bomb when ready to end game)/multibomb (so you always have the bomb without finding the last spot it was or seeing BOMB appear on your screen)/ Killcams disabled (so you don't have to wait after each kill).
Then, take your character to the enemies spawn and gun them down. As they die, they will respawn within 20 feet, some spawns are even in a line, making this even easier (Favela by graves, Terminal by the plane). Some spawns are inside (Vacant/Terminal each have one), choose your map or team accordingly.
Before doing this, see what attachments/number of kills are needed for each gun/class/perk that you make, then write this information down so you can't forget.
For Grenade Launchers, a direct impact will count, so just shoot them at close range to get it done quickly.
Rack up killstreaks, die, rack up more, use them when ready to unlock challenges for them too.
The Cold Blooded glitch still works, so use that to help unlock CB Pro. (shoot heli until it spins out of control, then each bullet that hits it will count as another kill for CB>CB Pro).
Switch out with the enemy characters, so you can get your fair share of shooting down killstreaks.
At first, I recommend using Care Package and Emergency Air Drop to get the most killstreaks that you can, rack up a bunch and call them all in. Be careful, if you take too long picking up your 30+ crates, they will eventually disappear. You can always have an enemy pick up helicopters and such so that you can shoot them down for the CB challenges.
Have the enemies drop Tactical Insertions (or claymores, but TI's are less dangerous)and shoot them for Sit Rep Pro.
After calling in helicopters or setting several Sentry Guns, run around with Marathon and Lightweight to kill time and fulfill their perk challenges. Usually, 2 Sentry Guns at a time is enough, replace them as they time out.
Don't use "headshots only" as the sentry guns and 'copters have trouble shooting people (they like to shoot feet).
This takes some time, but much less than regular levelling up and it is very zen.
Have fun split-screening.
[more cheats for Call of Duty: Modern Warfare 2 here]