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.
Finished the 9th mission on Legendary with the Scorpion intact.
Earned a First Strike Medal in a Matchmaking game.
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.
Hijacked a Banshee during the Reach Campaign.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
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.
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.
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.
Earned a Killing Spree in multiplayer Matchmaking.
Completed all of the Daily Challenges in a given day.
Won a game of Invasion in the 1st phase.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Completed the 4th mission on Normal or harder.
Completed the 3rd mission on Normal or harder.
Performed an Assassination on an enemy.
Traded weapons with an AI ally in Campaign.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Reached the rank of Corporal in the UNSC.
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.
Used the File Browser to upload a file to your File Share.
As an Elite, killed 5 Spartan players in Matchmaking.
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.
Recommended a file to someone.
Scored 1,000,000 points in a Firefight game.
Earned a Killionaire medal in Firefight.
Completed a Firefight set on Legendary without dying.
Scored 200,000 points in a Firefight game.
Scored 50,000 points in a Firefight game.
Noble 1: Destroyed a vehicle using Armor Lock in a matchmade 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: Kill a player at long range with the DMR in a matchmade Slayer game.
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: Successfully return 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.