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