There are nine achievements tied to campaign missions - one for completing each mission (save for Lone Wolf) on the Normal difficulty level or higher. These achievements can be earned solo or co-op.
We're Just Getting Started
Complete the 2nd mission on Normal or harder.
Protocol Dictates Action
Complete the 3rd mission on Normal or harder.
I Need A Weapon
Complete the 4th mission on Normal or harder.
Complete the 5th mission on Normal or harder.
You Flew Pretty Good
Complete the 6th mission on Normal or harder.
Into The Howling Dark
Complete the 7th mission on Normal or harder.
Dust And Echoes
Complete the 8th mission on Normal or harder.
This Is Not Your Grave
Complete the 9th mission on Normal or harder.
Send Me Out... With A Bang
Complete the 10th mission on Normal or harder.
There are four achievements tied to difficulty levels. The first three can be fulfilled solo or co-op, but 'A Monument To All Your Sins' must complete the campaign on Legendary difficulty level - alone. Good luck!
Note that these achievements stack, meaning if you beat the campaign solo or co-op on Legendary your first run through the game, you would unlock 'A Monument To All Your Sins' or 'Gods Must Be Strong' as well as the achievements for completing the game on the Heroic and Normal difficulty levels.
The Soldier We Need You To Be
Complete the Campaign on Normal difficulty.
Folks Need Heroes...
Complete the Campaign on Heroic difficulty.
Gods Must Be Strong
Complete the Campaign on Legendary difficulty.
A Monument To All Your Sins
Complete every mission in Halo: Reach on Legendary, alone.
There are a handful of achievements unlocked by doing something special during certain missions in the campaign. Note that the opening video sequence, 'Noble Actual,' is considered the first mission, so use the list below to ensure you're attempting to meet the requirement of an achievement on the correct mission.
1st Mission: Noble Actual
2nd Mission: Winter Contingency
3rd Mission: ONI Sword Base
4th Mission: Nightfall
5th Mission: Tip of the Spear
6th Mission: Long Night of Solace
7th Mission: Exodus
8th Mission: New Alexandria
9th Mission: The Package
10th Mission: The Pillar of Autumn
KEEP IT CLEAN
Kill 7 Moa during the 2nd mission of the Campaign.
Moa are large, ostrich-like creatures indigenous to Reach. Hunt seven of these on Winter Contingency and you'll unlock this achievement. Moa can be elusive, but they don't disappear, so keep after them. Keep an eye out for Moa near the start of the level, while facing the first batch of Elites, and while clearing the landing zone.
All in all, Moa are quite common, so this isn't a difficult achievement to unlock. Just keep an eye out and do note that running over a Moa with a vehicle won't count as a kill.
Hunt the indigenous Moa on Winter Contingency to unlock this achievement.
They've Always Been Faster
Clear the 2nd mission without setting foot in a drivable vehicle.
Stay away from the civilian trucks in Winter Contingency and go at it on foot. As long as you stay out of the trucks, you'll unlock this achievement once you reach the Falcon.
Two Corpses In One Grave
Kill 2 vehicles at once with the Target Locator in the 3rd mission.
At the start of 'Get the Hell Off My Lawn' on ONI: Sword Base (load Rally Point Alpha on the mission select screen), grab the Target Locator the onscreen Ordnance marker is pointing to and head through the gate. Head to the rock over on the left and two Wraiths will soon appear in the distance.
With the Target Locator equipped, click the Right Stick to zoom in, set the reticule between the two Wraiths and hold RT to set the marker. With any luck, the two Wraiths will converge inside the red circle and both get pummelled by missiles.
Earning this achievement requires a bit of luck, as sometimes the Wraiths won't act the way you want them to. If you fail to destroy both Wraiths, pause the game and select 'Revert to Last Save' to try again.
Unlocking Two Corpses In One Grave
Banshees, Fast And Low
Hijack a Banshee during the Reach Campaign.
The best opportunity to unlock this achievement comes near the end of Tip of the Spear. When the large Covenant spire inside the shield comes into view, there should be a Banshee flying low near the ridge you're standing on.
Pick up a Plasma Pistol from one of the Grunts you killed in the previous area and fire a charged shot at the low-flying Banshee. If you're successful and the Banshee falls to the ground, immediately move in and hold X to hijack it. If the Banshee lands far from you, charge another Plasma Pistol shot while moving toward the downed Banshee and hit it again before it's able to take off.
Unlocking Banshees, Fast And Low
Your Heresy Will Stay Your Feet
Kill the Elite Zealot before he escapes during the 5th mission.
You'll encounter the Elite Zealot on Tip of the Spear shortly after Rally Point Alpha. You must kill this Elite before he escapes to earn this achievement. You can accomplish this with a Plasma Pistol and a Needle Rifle or DMR. Make sure you're playing on the Normal difficulty level or higher.
Fight your way across the bridge and around the side of the factory. Pick up a Plasma Pistol on your way, as well as a Needle Rifle dropped by a Skirmisher or Jackal. You'll eventually come to some stairs with a weapon rack at the top. The Elite Zealot will be in the large area beyond these stairs with a red arrow above his head. Hit the Elite Zealot with a charged Plasma Pistol shot to disable his shield, then equip your Needle Rifle or DMR and shoot the Zealot once in the head to put him down.
Unlocking Your Heresy Will Stay Your Feet
Wake Up Buttercup
Destroy the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Unlocking this achievement requires destroying all four of the Corvette's engines and all of the Corvette's escorts on the Long Night of Solace mission in under three minutes. Ensure you're playing on the Heroic difficulty level or higher when attempting this.
This achievement is difficult to unlock solo, so enlist the help of a friend or two for an easier time. Start by flying behind the Corvette and blasting each of its four engines with your Sabre's missiles. Once you've managed that, turn your attention to the Banshee and Seraph escorts.
Banshees can be easily destroyed with missiles. If you were unaware, you can switch to your Sabre's missiles by hitting the Y button. To shoot down a Banshee, keep the target reticule trained on your target until the diamond marker forms and turns red. Then, press RT to fire the missiles and hopefully hit the mark.
Destroy the Corvette's engines and all of its escorts in under three minutes on Heroic or harder to unlock an achievement.
To down a Seraph, you first must disable its shield. This can be accomplished using the Sabre's machine guns. When you spot a Seraph, ensure you have machine guns set and hold RT while keeping the target reticule trained on the Seraph. When you see the shield go down, hit Y to switch to missiles, then lock on to the Seraph and fire to shoot it down.
You can easily track the escorts by following the markers that appear on screen. Get close enough and your target will be tagged, so you won't have to make out which type of ship you're chasing.
If They Came To Hear Me Beg
Perform an Assassination against an Elite to survive a fall that would've been fatal.
The best spot to earn this achievement is at the beginning of The Pillar of Autumn mission. At the start, move to the mountainside up ahead and look for an Elite standing below. Then, back up, hit LB to trigger Sprint and jump off the cliff and toward the Elite.
The important step here is to hit LB again while in mid-air to disable Sprint, otherwise you won't get the achievement even if you manage to assassinate the Elite. So, hit LB after jumping to switch off Sprint, then hold RB when you reach the Elite to perform the assassination and save yourself from a messy death.
Unlocking If They Came to Hear Me Beg
I Didn't Train To Be A Pilot
Killed 3 of the anti-aircraft batteries during the 8th mission.
You'll definitely earn this achievement by completing New Alexandria if you're playing solo, as you'll face at least five turrets at some point in the mission. If you played through the mission on co-op and didn't unlock this one, just run through it solo (on the Normal difficulty level or higher).
Tank Beats Everything
Finished the 9th mission on Legendary with the Scorpion intact.
This is a tough achievement if you're playing solo. As such, it's best to enlist the help of a few friends to get this one done. Note that you must be playing on the Legendary difficulty level to unlock this achievement.
It's tough to keep the tank intact, so one trick is to leave the tank where it is and go at it on foot. If you need the tank's firepower, you can have one player sit in the tank and the other three keep ahead and act as escorts.
Reach Sword Base with the Scorpion intact on Legendary to unlock this one.
You just need to keep the tank intact until you reach Sword Base. Once you reach Sword Base, the achievement should unlock.
Earn a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
You must score a Triple Kill while 'Jetpacking,' which means while boosting. A Triple Kill while falling after boosting with the Jetpack won't unlock this achievement.
This achievement can be unlocked during the campaign (at any point the Jetpack armour ability is available), but doing so requires some luck.
By far the easiest way to earn this achievement is during a Rocket Fight Firefight match. Choose the Jetpack armour ability at the start and move close to one of the first Phantom drop ships that flies in to drop off troops. When the doors open and the Grunts start to pile out, hold LB to boost and be ready to shoot at the Grunts when they land bunched together..
Earning the Lucky Me Achievement
Use a Health Pack to replenish life after taking body damage.
When your Spartan has taken actual damage (not just a disabled shield), approach a Health Pack and hold the X button to use it to restore health. Needless to say, you'll earn this achievement fairly early.
That's A Knife
Perform an Assassination on an enemy.
To perform an assassination, move behind an enemy and hold the RB button. If you just press RB, you'll perform a one-hit pummel and not an assassination. For the kill to count as an assassination, an animation must accompany it.
You can earn this achievement in any mode, but if you want to get this one out of the way, load up the Nightfall campaign mission and assassinate the oblivious Elite at the start.
Performing an Assassination
I See You Favour A .45
Kill 10 enemies in a Firefight or Campaign session with the M6G pistol.
The M6G pistol (also known as the Magnum) is a powerful weapon, so this is an easy achievement. Just play through a campaign mission and use your Magnum to headshot Grunts. The Magnum is also great for putting down an Elite once you've disabled his shield with a charged Plasma Pistol shot.
An Elegant Weapon
Kill 10 enemies in a Firefight or Campaign session with the DMR.
The DMR is one of the most effective weapons in the campaign mode. Once you get your hands on one, you should have this achievement in no time. As with the Magnum, the DMR is great for headshotting Grunts, as well as finishing off an Elite once you've disabled his shield with a charged Plasma Pistol shot.
A Spoonful Of Blamite
Kill 10 enemies in Firefight or Campaign with a supercombine explosion.
You must use a Needler or Needle Rifle (or both) to earn 10 Needle Kill medals in one Firefight or Campaign mission session to earn this achievement. The easiest way to do this is with a Needle Rifle, but the Needler works just as well.
To kill an enemy with a supercombine explosion, keep shoot needles at his body until the needles explode and kill him. If you're going for this achievement and are using a Needle Rifle, don't go for headshots until you've got it, as those kills won't count.
Trade weapons with an AI ally in Campaign.
You can't trade weapons with your Noble Team allies, but you can with any Marine. This is often a good strategy if a Marine has a weapon you'd like, such as a DMR. As well, you can also give a Marine a powerful weapon such as a Fuel Rod Cannon and let him or her go crazy with infinite ammo.