A fast way to earn this quickly is to search on the matchmaking menu for players of Level 69 and above. Virtually every player at this level is farming Crawmerax the Invincible for his loot drops and massive XP awards. Even in the 40s, I was levelling up twice per kill of Crawmerax courtesy of those other players. I would advise this only if you don't want to take your time and get this naturally while achieving all other content in the add-ons, but this is much faster than even shooting guardians with the shock sniper outside of the Vault.