Heres a way to gain quick XP...

[wheresurh3ad]

Start a game of Retrieval with a buddy or using another account and xbox on "Silo", make sure you use the Perk that doubles your xp. Now everytime you capture the canister you will get 100 xp a time, ive gotten as much as 1570xp in 10 minutes i believe it was and that was with some random guy joining later in the game.This is still time consuming but i believe it is the quickest method...