Story-related achievements will be unlocked automatically by completing the campaign missions in varying difficulty. These are pretty much self-explanatory so no further explanation will be needed for them. Explanation and videos will be provided to the other achievements.
Back in the Fight (5 points)
Start the Single Player Campaign on any difficulty.
Too Big to Fail (10 points)
Destroy the Jamming Tower. Complete "Black Tuesday" on any difficulty.
Wet Work (10 points)
Take back New York Harbor. Complete "Hunter Killer" on any difficulty.
Carpe Diem (10 points)
Escape the mountain safe house. Complete "Persona Non Grata" on any difficulty.
Frequent Flyer (10 points)
Defend the Russian President. Complete "Turbulence" on any difficulty.
Up to No Good (10 points)
Infiltrate the village. Complete "Back on the Grid" on any difficulty.
One Way Ticket (10 points)
Make it to Westminster. Complete "Mind the Gap" on any difficulty.
Welcome to WW3 (10 points)
Save the US Vice President. Complete "Goalpost" on any difficulty.
Sandstorm! (10 points)
Assault the shipping company. Complete "Return to Sender" on any difficulty.
Back Seat Driver (10 points)
Track down Volk. Complete "Bag and Drag" on any difficulty.
We'll Always Have Paris (10 points)
Escape Paris with Volk. Complete "Iron Lady" on any difficulty.
Vive la Révolution! (10 points)
Reach the church. Complete "Eye of the Storm" on any difficulty.
Requiem (10 points)
Escape the city. Complete "Blood Brothers" on any difficulty.
Storm the Castle (10 points)
Discover Makarov's next move. Complete "Stronghold" on any difficulty.
Bad First Date (10 points)
Find the girl. Complete "Scorched Earth" on any difficulty.
Diamond in the Rough (10 points)
Rescue the Russian President. Complete "Down the Rabbit Hole" on any difficulty.
The Big Apple (25 points)
Complete "Black Tuesday" and "Hunter Killer" on Veteran difficulty.
Out of the Frying Pan… (25 points)
Complete "Persona Non Grata", "Turbulence", and "Back on the Grid" on Veteran difficulty.
Payback (25 points)
Complete "Mind the Gap", "Goalpost", and "Return to Sender" on Veteran difficulty.
City of Lights (25 points)
Complete "Bag and Drag" and "Iron Lady" on Veteran difficulty.
The Darkest Hour (25 points)
Complete "Eye of the Storm", "Blood Brothers", and "Stronghold" on Veteran difficulty.
This is the End (25 points)
Complete "Scorched Earth", "Down the Rabbit Hole", and "Dust to Dust" on Veteran difficulty.
Who Dares Wins (40 points)
Complete the campaign on any difficulty.
The Best of the Best (100 points)
Complete the campaign on Hardened or Veteran difficulty.
Strike! (20 points)
Kill 5 enemies with a single grenade in Single Player or Special Ops.
The method below took me several attempts since the timing of cooking the grenades, throwing it and the angle of the throw needs to be just right. I found a Youtube video showing a much easier method of blowing yourself up in the middle of a group of enemies in the same stage. Just search for it if you want to try it.
Jack the Ripper (20 points)
Melee 5 enemies in a row in Single Player or Special Ops.
This is easier to get done by doing a mission in Recruit difficulty. Just don't fire a single shot and keep melee-ing the enemies.
Informant (20 points)
Collect 22 Intel Items. (See Squad Leader)
Scout Leader (35 points)
Collect 46 Intel Items. Please see our Intel Location guide: http://www.supercheats.com/guides/call-of-duty-modern-warfare-3/intel-location-guide
This Is My Boomstick (20 points)
Kill 30 enemies with the XM25 in "Black Tuesday."
Your XM25 can deal splash damage so you can take out multiple enemies who are grouped together. The best spot to kill as many soldiers as possible early on is right before you enter the first building. There's a large enemy force there which you should run away from as part of the story. You can see the same group from the window, as you fight your way to the 2nd floor. Just wait for at least 3-4 soldiers get close before firing in their middle. I got this achievement unlocked by killing a couple more enemies in the trading floor of the Foreign Exchange building.
What Goes Up… (20 points)
Destroy all the choppers with only the UGV's grenade launcher in "Persona Non Grata."
Don't fire a single bullet to the choppers as this will lost your chance in unlocking the achievement. See the video below:
What goes up, must go down - using grenades
For Whom the Shell Tolls (20 points)
Destroy all targets during the mortar sequence with only 4 shells in "Back on the Grid."
To unlock the achievement for this sequence, you'll have to destroy four targets with only four shells. The first three are technicals so you wouldn't have problems targeting them. Take note that the mortar doesn't have that much splash damage so you have to land a direct hit to be able to kill them. The last target is a moving foot soldier so you have to time your release so it he gets hit directly. Otherwise, he'll just be blown like a ragdoll and still live, missing your chance to get this achievement. Just restart the checkpoint to do it again.
It's raining mortars!
Kill Box (20 points)
Kill 20 enemies with the Chopper Gunner in a single run in "Return to Sender."
This is preferably easier done when you need to take out the MG nests in the mansion since the enemies are close to each other.
Like picking off ants who fire back
Danger Close (20 points)
Take down a chopper with an AC-130 smoke grenade in "Bag and Drag."
After getting the ability to thrown support marker smoke grenades, you'll have to move to a plaza. There will be an enemy helicopters that will drop down reinforcements. Toss a marker grenade directly underneath it, preferably between the ropes and wait for the AC-130 to bring it down. You need to do this quick. If you failed, just reload the checkpoint. If there are a lot of enemies preventing your progress, lower the difficulty.
It's really dangerously close
Ménage à Trois (20 points)
Destroy 3 tanks with a single 105mm shot in "Iron Lady."
You need to wait for the 3 tanks to stand close to each other before firing a 105mm round in the middle.
Nein (20 points)
Kill 9 enemies with A-10 strafing runs in "Scorched Earth."
50/50 (20 points)
Complete a Special Ops Mission Mode game with the same number of kills as your partner.
The first Spec-Ops training mission has an equal number of enemies. You can take your time with a partner and count your “kills” manually to unlock this.
Birdie (20 points)
Kill 2 enemy helicopters without getting hit in a Special Ops Survival game.
Got this in the “Resistance” map. When the two enemy helicopters appear (Wave 6). When they appear on the map, you should be already inside a cover, preferably a tunnel so you can move easily. You can have a partner or do it solo. Your partner can take the grunt and deal additional damage. As long as you deal the finishing blow, it should count as your kill.
Serrated Edge (15 points)
Finish a Juggernaut with a knife in Special Ops.
The Juggernaut is the heavily armed soldier which will appear in Survival Mode. This can be easily done with a partner (to act as a decoy) and some flashbangs. Just deal as much damage as possible and when you feel that he's softened up, get behind him and knife him. You can also throw a flash grenade to give yourself a few seconds window to get close and knife him.
Arms Dealer (20 points)
Buy all items from the Survival Weapon Armory.
First of all, you need to reach the appropriate level so the other weapons can be unlocked. This is not something you can unlock early on. You just need to buy all weapons, including the starter weapons (Five-Seven), including the ones you pick up. This doesn't have to be done in a single session though. The achievement is cumulative. Just concentrate on buying the weapon set you can afford. It will help if you'll manually list the items you have bought. For the attachments, buying one for any gun will “count” for other guns as well.
Danger Zone (20 points)
Buy all items from the Survival Air Support Armory.
Aside from allied support units (Delta and Riot Shield Squad) and Predator/Air Strike, you need to purchase all the Care packages as well.
Defense Spending (20 points)
Buy all items from the Survival Equipment Armory.
First, you must have unlocked all items before you can purchase them. The last item you can unlock is the Riot Shield and that's at LV37.
Get Rich or Die Trying (25 points)
Have $50,000 current balance in a Special Ops Survival game.
This can be boosted with a partner. Just pick up weapons and don't buy anything for the first few rounds. You can also have your partner send money to you.
I Live (10 points)
Survive 1 wave in a Special Ops Survival game.
Survivor (20 points)
Reach Wave 10 in each mission of Special Ops Survival mode.
Unstoppable (40 points)
Reach Wave 15 in each mission of Special Ops Survival mode.
Memorize the maps and consider finding “defensive” points. These are parts of the maps which you can easily dig in and hold out.
Use turrets. You must place them in an elevated position where they can provide maximum cover and avoid getting destroyed easily
Reinforcements – The Riot Shield team is always a better choice than the Delta Squad. Having them around increases your chances to survive.
Claymores – Place them along the paths the enemies are most likely to take.
Partner – Having a human partner will give you the advantage of having one additional turret and reinforcement squad.
No Assistance Required (20 points)
Complete a Special Ops Mission Mode game on Hardened or Veteran with no player getting downed.
Brag Rags (10 points)
Earn 1 star in Special Ops Mission Mode.
Tactician (20 points)
Earn 1 star in each mission of Special Ops Mission Mode.
Overachiever (40 points)
Earn 48 stars in Special Ops Mission Mode.
Flight Attendant (20 points)
Kill all 5 enemies during the zero-g sequence in "Turbulence."