Big Spender Achievement
Spend 10,000 credits on loadout upgrades
Before each mission you are allowed to modify your loadout. You can earn credits by completing missions and Assault Ops. You can also get credits for completing campaigns in Halo: The Master Chief collection. You get 3,000 credits per campaign completed, so if you beat all 4 you get 12,000 which is more than enough to unlock this achievement.
If you want to quickly obtain this achievement and for some reason don't feel like playing, simply select a mission, spend as many credits as possible, then abort the mission once it loads up.