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