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.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Completed every mission in Halo: Reach alone, on Legendary.
Completed the Campaign on Legendary difficulty.
Finished the 9th mission on Legendary with the Scorpion intact.
Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Completed a Weekly Challenge.
Completed all of the Daily Challenges in a given day.
Purchased an item from the Armory.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Completed the Campaign on Heroic difficulty.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
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.
Earned a Killionaire medal in Firefight.
Scored 50,000 points in a Firefight game.
Advanced a Commendation to a Silver state.
As an Elite, killed 5 Spartan players in Matchmaking.
Won a game of Invasion in the 1st phase.
Used the File Browser to upload a file to your File Share.
Hijacked a Banshee during the Reach Campaign.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
Earned a First Strike Medal in a Matchmaking game.
Recommended a file to someone.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
Completed the 10th mission on Normal or harder.
Completed the Campaign on Normal difficulty.
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 7th mission on Normal or harder.
Earned a Killing Spree in multiplayer Matchmaking.
Reached the rank of Corporal in the UNSC.
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.
Completed the 3rd mission on Normal or harder.
Traded weapons with an AI ally in Campaign.
Completed the 2nd mission on Normal or harder.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
Performed an Assassination on an enemy.
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: 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 Double Kill with the shotgun in multiplayer Matchmaking.
Noble 1: Earn a Double Kill from the Grave in multiplayer Matchmaking.
Noble 1: Collect all of the flags in a matchmade Stockpile game.
Noble 1: Earn a kill in a matchmade Rocket Race game.
Noble 1: Blow 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: Prevent the attacking team from touching the flag in a matchmade 1-Flag CTF game.
Defiant: Assassinate a player that just exited Armor-Lock in a matchmade game.
Defiant: Be the last surviving uninfected player in a matchmade Infection 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: Infect 3 players in a single round in a matchmade Infection game.
Anniversary: Stick and kill the flag carrier with a Plasma Grenade in a matchmade game.
Anniversary: Perform a Beat Down on an enemy player while using the Jetpack during a matchmade game.
Anniversary: Run over 5 people with a vehicle in a matchmade game.
Anniversary: Capture 10 skulls at once in a matchmade Headhunter game.