Here is the full list of all 20 Spartan Firefight achievements.
Reach your first rank.
Reach the rank of Sergeant.
Reach the rank of Major.
Reach the rank of General.
Complete all of the Daily Challenges in a given day.
Upgrade any Weapon for the first time in the armory.
Fully upgrade any Weapon in the armory.
Buy a Skin for any weapon in the shop.
Buy an Emblem in the shop for the first time.
Get an Invincible medal in any game mode (100 kills without dying).
Advance a Commendation to a Gold state.
Score 50,000 points in a Firefight game.
Score 25,000 points in an Floods game.
Earn a Triple Kill with one shot of the Spartan Laser in a Red vs Blue game.
Earn a Double Kill with the Shotgun in a Red vs Blue game.
Earn a Killing Spree medal in a Red vs Blue game.
Earn 3 stars on any mission.
Complete every mission on any difficulty.
Earn 3 stars on every mission.
Reach the rank of 5-Star General.