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.
Completed every mission in Halo: Reach alone, on Legendary.
Completed the Campaign on Legendary difficulty.
Reached the rank of Captain in the UNSC.
Won a game of Invasion in the 1st phase.
As an Elite, killed 5 Spartan players in Matchmaking.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
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.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Hijacked a Banshee during the Reach Campaign.
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 Heroic difficulty.
Completed the Campaign on Normal difficulty.
Completed the 9th mission on Normal or harder.
Completed all of the Daily Challenges in a given day.
Advanced a Commendation to a Silver state.
Completed the 8th mission on Normal or harder.
Killed 3 of the anti-aircraft batteries during the 8th mission.
Completed a Weekly Challenge.
Completed the 7th mission on Normal or harder.
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.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Used the File Browser to upload a file to your File Share.
Scored 50,000 points in a Firefight game.
Recommended a file to someone.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Earned a Killing Spree in multiplayer 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.
Completed the 4th mission on Normal or harder.
Reached the rank of Corporal in the UNSC.
Earned a First Strike Medal in a Matchmaking game.
Earned a Killionaire medal in Firefight.
Performed an Assassination on an enemy.
Completed the 3rd mission on Normal or harder.
Traded weapons with an AI ally in Campaign.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
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: Earned a kill in a matchmade Rocket Race game.
Noble 1: Killed a player at long range with the DMR in a matchmade Slayer game.
Noble 1: Your team collected all of the flags in a matchmade Stockpile game.
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: Earn 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: Returned two flags in a matchmade CTF game.
Defiant: Kill 3 players with the Oddball in a matchmade Oddball game.
Defiant: Capture two flags from an enemy capture zone in a matchmade Stockpile 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: Get 3 headshot kills 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.
Anniversary: Kill an enemy with their own Plasma Grenade that is stuck to you in a matchmade game.