Full list of all 69 Halo: Reach achievements worth 1,700 gamerscore. It takes around 40-50 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 49 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 20 achievements worth 700 Gamerscore.
Purchased an item from the Armory that required the rank of Lt. Colonel.
Reached the rank of Captain in the UNSC.
Earned a Killing Spree in multiplayer Matchmaking.
Completed all of the Daily Challenges in a given day.
Won a game of Invasion in the 1st phase.
Earned a First Strike Medal in a Matchmaking game.
Scored 1,000,000 points in a Firefight game.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Recommended a file to someone.
Used the File Browser to upload a file to your File Share.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Scored 200,000 points in a Firefight game.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Completed a Firefight set on Legendary without dying.
Earned a Killionaire medal in Firefight.
Scored 50,000 points in a Firefight game.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
As an Elite, killed 5 Spartan players in Matchmaking.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Hijacked a Banshee during the Reach Campaign.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
Completed a Weekly Challenge.
Completed the 10th mission on Normal or harder.
Completed every mission in Halo: Reach alone, on Legendary.
Completed the Campaign on Legendary difficulty.
Completed the Campaign on Heroic difficulty.
Completed the Campaign on Normal difficulty.
Advanced a Commendation to a Silver state.
Completed the 9th mission on Normal or harder.
Finished the 9th mission on Legendary with the Scorpion intact.
Completed the 8th mission on Normal or harder.
Killed 3 of the anti-aircraft batteries during the 8th mission.
Completed the 7th mission on Normal or harder.
Completed the 6th mission on Normal or harder.
Completed the 5th mission on Normal or harder.
Killed the Elite Zealot before he escaped during the 5th mission.
Completed the 4th mission on Normal or harder.
Purchased an item from the Armory.
Reached the rank of Corporal in the UNSC.
Performed an Assassination on an enemy.
Completed the 3rd mission on Normal or harder.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
Completed the 2nd mission on Normal or harder.
Traded weapons with an AI ally in Campaign.
Used a Health Pack to replenish life after taking body damage.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Noble 1: Earned a Double Kill from the Grave in multiplayer Matchmaking.
Noble 1: Destroyed a vehicle using Armor Lock in a matchmade game.
Noble 1: Killed a player at long range with the DMR in a matchmade Slayer game.
Noble 1: Earned a kill in a matchmade Rocket Race game.
Noble 1: Earned a Double Kill with the shotgun in multiplayer Matchmaking.
Noble 1: Your team collected all of the flags in a matchmade Stockpile game.
Noble 1: Your team blew up the research facility in a matchmade Invasion game on Breakpoint.
Defiant: Captured two flags from an enemy capture zone in a matchmade Stockpile game.
Defiant: Returned two flags in a matchmade CTF game.
Defiant: Killed 3 players with the Oddball in a matchmade Oddball game.
Defiant: Assassinated a player that just exited Armor-Lock in a matchmade game.
Defiant: Was the last surviving uninfected player in a matchmade Infection game.
Defiant: Prevented the attacking team from touching the flag in a matchmade 1-Flag CTF game.
Anniversary: Killed the flag carrier with a Plasma Grenade in a matchmade game.
Anniversary: Performed a Beat Down on an enemy while using the Jetpack during a matchmade game.
Anniversary: Ran over 5 people with a vehicle in a matchmade game.
Anniversary: Captured 10 skulls at once in a matchmade Headhunter game.
Anniversary: Killed an enemy with their own grenade that was stuck to you in a matchmade game.
Anniversary: Earned 3 headshots in a row in a matchmade game.
Anniversary: Infected 3 players in a single round in a matchmade Infection game.