Don't think this was posted before, so here it is!
( Note, this is not a glitch, its a challenge )
1. Get Dead Silence Pro ( 70 Close range kills I think. )
2. Look for the challenge Featherweight in the perks category, you should have it unlocked.
3. Find a match and when you get in the game, find a staircase/platform that would hurt you if you jumped off it.
4. Get up to the staircase/platform, jump down, get back up and repeat!
The Featherweight challenge awards you with XP every couple times you jump off a platform that would injure you since Dead Silence Pro is "No Falling Damage".
So just keep doing that and every couple times you jump off ( I think it is 2, 10,25,50,100) , it will give you XP since you complete the challenge!
Have fun guys!