Halo: Reach Cheats

Your Heresy Will Stay Your Feet (25): Killed the Elite Zealot before he escaped during the 5th mission.
If They Came To Hear Me Beg (25): Performed an Assassination against an Elite to survive a fall that would've been fatal.
Wake Up Buttercup (25): Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Tank Beats Everything (25): Finished the 9th mission on Legendary with the Scorpion intact.
Lucky Me (25): Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
KEEP IT CLEAN (5): Killed 7 Moa during the 2nd mission of the Campaign.
I Didn't Train To Be A Pilot (10): Killed 3 of the anti-aircraft batteries during the 8th mission.
Doctor, Doctor (5): Used a Health Pack to replenish life after taking body damage.
That's A Knife (10): Performed an Assassination on an enemy.
I See You Favour A .45 (10): Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
An Elegant Weapon (10): Killed 10 enemies in a Firefight or Campaign session with the DMR.
Swap Meet (10): Traded weapons with an AI ally in Campaign.
A Spoonful Of Blamite (10): Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Be My Wingman Anytime (5): Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Yes, Sensei (10): Earned a First Strike Medal in a Matchmaking game.
Skunked (10): Won a game of Invasion in the 1st phase.
What's A Killing Spree? (5): Earned a Killing Spree in multiplayer Matchmaking.
Crowd Control (10): Earned a Killionaire medal in Firefight.
Knife To A Gun Fight (5): As an Elite, killed 5 Spartan players in Matchmaking.
Score Attack (10): Scored 15,000 points in Score Attack Firefight Matchmaking.
Firestarter (10): Scored 50,000 points in a Firefight game.
Blaze Of Glory (25): Scored 200,000 points in a Firefight game.
Heat In The Pipe (75): Scored 1,000,000 points in a Firefight game.
Game, Set, Match (25): Completed a Firefight set on Legendary without dying.
Make It Rain (10): Purchased an item from the Armory that required the rank of Lt. Colonel.
The Start Of Something (15): Reached the rank of Corporal in the UNSC.
An Honor Serving (25): Reached the rank of Captain in the UNSC.
A Storage Solution (5): Used the File Browser to upload a file to your File Share.
A New Challenger (10): Completed all of the Daily Challenges in a given day.
Make It Drizzle (10): Purchased an item from the Armory.
Cool File, Bro (5): Recommended a file to someone.
Lemme Upgrade Ya (10): Advanced a Commendation to a Silver state.
One Down, 51 To Go (10): Completed a Weekly Challenge.

Noble Map Pack DLC Achievements
Both Barrels (50): Noble 1: Earn a Double Kill with the shotgun in multiplayer Matchmaking.
Cross-Mappin' (50): Noble 1: Kill a player at long range with the DMR in a matchmade Slayer game.
Offensive Driver (20): Noble 1: Earn a kill in a matchmade Rocket Race game.
Poppin' & Lockin' (25): Noble 1: Destroy a vehicle using Armor Lock in a matchmade game.
Totally Worth It (50): Noble 1: Earn a Double Kill from the Grave in multiplayer Matchmaking.
You Ate All the Chips (42): Noble 1: Collect all of the flags in a matchmade Stockpile game.
You Blew It Up! (13): Noble 1: Blow up the research facility in a matchmade Invasion game on Breakpoint.

Defiant Map Pack Achievements
Candy from a Baby (75): Capture two flags from an enemy capture zone in a matchmade Stockpile game.
Crackin' Skulls (45): Kill 3 players with the Oddball in a matchmade Oddball game.
Return to Sender (30): Successfully return two flags in a matchmade CTF game.

Noble Map Pack Achievements
Both Barrels (50): Earn a Double Kill with the shotgun in multiplayer Matchmaking.
Cross-Mappin' (50): Kill a player at long range with the DMR in a matchmade Slayer game.
Offensive Driver (20): Earn a kill in a matchmade Rocket Race game.
Poppin' & Lockin' (25): Destroy a vehicle using Armor Lock in a matchmade game.
Totally Worth It (50): Earn a Double Kill from the Grave in multiplayer Matchmaking.
You Ate All the Chips (42): Collect all of the flags in a matchmade Stockpile game.
You Blew It Up! (13): Blow up the research facility in a matchmade Invasion game on Breakpoint.

Anniversary Map Pack Achievements
Bounty Hunter (75): Anniversary: Captured 10 skulls at once in a matchmade Headhunter game.
Dive Bomber (50): Anniversary: Performed a Beat Down on an enemy while using the Jetpack during a matchmade game.
Emergency Room (15): Anniversary: Infected 3 players in a single round in a matchmade Infection game.
From Hell's Heart (25): Anniversary: Killed an enemy with their own grenade that was stuck to you in a matchmade game.
License to Kill (15): Anniversary: Ran over 5 people with a vehicle in a matchmade game.
Stick it to the Man! (20): Anniversary: Killed the flag carrier with a Plasma Grenade in a matchmade game.
Top Shot (50): Anniversary: Earned 3 headshots in a row in a matchmade game.

Avatar Awards
These awards can be used with your Avatar after meeting the requirements.

Carter's Helmet: Clear a Campaign mission on Legendary without dying.
Emile's Helmet: Earned a Bulltrue medal in either multiplayer or Firefight Matchmaking.
Jorge's Helmet: Earn a Killtacular in multiplayer Matchmaking.
Jun's Helmet: Kill 100 enemies in a row without dying in either the Campaign or Firefight.
Kat's Helmet: Avenged a teammate's death in multiplayer Matchmaking

Easy Avatar Carter's Helmet unlock
Normally, you'd have to complete a level on Legendary without dying to get this. However, you can play up until just before the end and "Save and Quit" on a checkpoint. Then, Resume the game and finish the level (without dying for those last few parts). After you finish, you should get the Avatar Unlock pop-up.