You can gain as much as 50,000 XP in an online match by doing the following. Get twelve people into a lobby and kick six of them out. Make sure the host of the lobby temporarily has a bad connection so that you will become the host. Find a Search And Destroy game. Once in the lobby, find six people to join. Make the connection good again and start the match. Choose a class that has the riot shield and use the One Man Army Perk it you have it unlocked. When the match begins, get another player to fire at you while you hold the riot shield. You will get 500 XP every few seconds.