Warhammer 40,000: Space Marine

This can be done in Private Matches, like almost all of the multiplayer achievements. This makes this ridiculously easy to boost, just going to take a while (Keeper of the Armory made easy!)

Set up a group of 9 friends and start a private match. Have 1 person be Space Marine, the other 8 will need to be Chaos Space Marine.

The host can adjust the time limit, health/armor limits, score limit, and spawn times.

Game Type: Seize Ground
Map: Waste Management
Time: 10:00 Minutes
Score: 2500
Spawn: Lowest Possible
Armor: 25%

Easiest challenges for a low level: Kills with Melee, As a Space Marine, As Chaos, etc... Look at the Challenges and press (Y) to get to the Armor Challenges. Knock out the first couple of challenges on a few different sets and this will unlock quickly. This will unlock naturally while going for Keeper of the Armory.

Once the match starts, have the Chaos Space Marines capture point B only. This will be the new spawn point for Chaos. Have the Space Marine start killing the Chaos just outside the ring.
Note, you want to spawn at Point B because it is faster than running back here every time from the Default Spawn.

With this setup, at 10 minutes the score for chaos will only be a little over 1000, so the game won't end early. However the Space Marine will get tons of kills, knocking out challenges very quickly.

Use this method and rotate every 10 Minutes for each person.

Note: If you finish the campaign, you will complete two armor challenges. To check which others you need, go to challenges and press the Y button.