- last post: 01.01.0001 12:00 AM PDT
Campaign, Firefight and any other Coop/Campaign modes only for achievements. Multiplayer ones always lead to boosting.
Completing the individual campaign missions
Complete the game on Heroic, Legendary and Mythic
Complete the game on Heroic, Legendary solo
Find all the skulls in campaign
Metagame scoring for the individual missions
Reach set?? round ?? wave ?? on Firefight
Find the skulls on all the multiplayer maps