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.
Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Scored 1,000,000 points in a Firefight game.
Completed a Firefight set on Legendary without dying.
Scored 200,000 points in a Firefight game.
Scored 50,000 points in a Firefight game.
Earned a Killionaire medal in Firefight.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Completed all of the Daily Challenges in a given day.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
Hijacked a Banshee during the Reach Campaign.
Won a game of Invasion in the 1st phase.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
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.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
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.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
As an Elite, killed 5 Spartan players in Matchmaking.
Earned a First Strike Medal in a Matchmaking game.
Completed the 4th mission on Normal or harder.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Earned a Killing Spree in multiplayer Matchmaking.
Used the File Browser to upload a file to your File Share.
Recommended a file to someone.
Reached the rank of Corporal in the UNSC.
Completed the 3rd mission on Normal or harder.
Performed an Assassination on an enemy.
Traded weapons with an AI ally in Campaign.
Completed the 2nd mission on Normal or harder.
Used a Health Pack to replenish life after taking body damage.
Purchased an item from the Armory.
Noble 1: Killed a player at long range with the DMR in a matchmade Slayer game.
Noble 1: Destroyed a vehicle using Armor Lock in a matchmade game.
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: Your team blew up the research facility in a matchmade Invasion game on Breakpoint.
Defiant: Returned two flags in a matchmade CTF game.
Defiant: Captured two flags from an enemy capture zone in a matchmade Stockpile game.
Defiant: Killed 3 players with the Oddball in a matchmade Oddball game.
Defiant: Prevented the attacking team from touching the flag in a matchmade 1-Flag CTF game.
Defiant: Was the last surviving uninfected player in a matchmade Infection game.
Defiant: Assassinated a player that just exited Armor-Lock 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: Ran over 5 people with a vehicle in a matchmade game.
Anniversary: Killed an enemy with their own grenade that was stuck to you in 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.