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.
Finished the 9th mission on Legendary with the Scorpion intact.
Completed all of the Daily Challenges in a given day.
Purchased an item from the Armory that required the rank of Lt. Colonel.
Completed a Weekly Challenge.
Reached the rank of Captain in the UNSC.
Completed every mission in Halo: Reach alone, on Legendary.
Completed the Campaign on Legendary difficulty.
Completed the Campaign on Heroic difficulty.
Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Earned a First Strike Medal in a Matchmaking game.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Recommended a file to someone.
Used the File Browser to upload a file to your File Share.
Completed the 7th mission on Normal or harder.
Completed the Campaign on Normal difficulty.
Advanced a Commendation to a Silver state.
Scored 1,000,000 points in a Firefight game.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Completed the 10th mission on Normal or harder.
Earned a Killing Spree in multiplayer Matchmaking.
As an Elite, killed 5 Spartan players in Matchmaking.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Won a game of Invasion in the 1st phase.
Completed a Firefight set on Legendary without dying.
Earned a Killionaire medal in Firefight.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
Scored 200,000 points in a Firefight game.
Scored 50,000 points in a Firefight game.
Hijacked a Banshee during the Reach Campaign.
Completed the 4th mission on Normal or harder.
Purchased an item from the Armory.
Completed the 9th mission on Normal or harder.
Completed the 8th mission on Normal or harder.
Killed 3 of the anti-aircraft batteries during the 8th mission.
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.
Used a Health Pack to replenish life after taking body damage.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Reached the rank of Corporal in the UNSC.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
Completed the 3rd mission on Normal or harder.
Traded weapons with an AI ally in Campaign.
Completed the 2nd mission on Normal or harder.
Performed an Assassination on an enemy.
Noble 1: Your team collected all of the flags in a matchmade Stockpile game.
Noble 1: Earned a kill in a matchmade Rocket Race game.
Noble 1: Earned a Double Kill from the Grave in multiplayer Matchmaking.
Noble 1: Earned a Double Kill with the shotgun 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: Your team blew up the research facility in a matchmade Invasion game on Breakpoint.
Defiant: Killed 3 players with the Oddball in a matchmade Oddball game.
Defiant: Captured two flags from an enemy capture zone in a matchmade Stockpile game.
Defiant: Returned two flags in a matchmade CTF game.
Defiant: Prevented the attacking team from touching the flag in a matchmade 1-Flag CTF 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.
Anniversary: Earned 3 headshots in a row in a matchmade game.
Anniversary: Infected 3 players in a single round in a matchmade Infection 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.