Sense they patched the glitch where you can't do snd in combat training I've been doing this the good ole hard way. This is how i make around 50-60k xp per game.

Map; Jungle
Mode; Team Deathmatch
CT Settings; 20Min - 300,000 Points, F1 - E9 - Recruit
Gun; GALIL - DUAL MAG
Equipment; FRAG - STUN - TACTICAL INSERTION
Perks; SCAVENGER - SLEIGHT OF HAND - MARATHON
Kill Streaks; UAV - CHOPPER GUNNER - DOGS

Tips; Start by going to the cliff in the map (AI can't climb the latter) Place TI down. Throw FRAG down by the bridge should get 3 frag kill, than pick off the other 2-3 that are right near them. Then pick off the 2 off on the left, than they should be coming by the bridge again. Camp a little until you get your dogs. Dogs help kill the left overs that you don't see with the chopper gunner. Than if your still alive jump off the cliff and re spawn with the TI.

1-11 Kills should take around 1-2Mins
11-45 Kills should take around 1min (With Chopper Gunner Dogs.)

Repeat this until you hit the score 300,000. You should have a K/D of 300-6, and a score of 50-60k every 20mins.

Use this method once someone finds out how to glitch the lobby back to snd