Edited for required XP for levels (I'll continue to update as I learn more...)
Level 31-- 173,640
Level 35 -- 263,200
Level 40 -- 552,400
Level 41 -- 671040 (armor unlocks)
Easily boosted in a private match with friends (the more the better).
Setup a Seize Ground private match, and adjust the game time to unlimited with a win score of 5000. Adjust the armor levels, spawn time, and health recharge to to your liking (as little as possible).
The 'earning' team should head over to the enemy spawn point, so ensure kills are quick and easy.
Then focus on weapon / armor challenges and work on some of the harder MP achievements. (ex. 150 kills with both pistols earns you 10,000 XP)
After about an hour we were able to unlock 'Defender', 'Devastation' (needs to be done in annihilation), 'Down to Earth', along with multiple weapon challenges.
There is no need to finish the match, your XP and earned items will be saved.
I left the match with a total of 27750 XP earned our first time boosting.
Hope this helps...