You'll unlock these achievements as you play the game. There are also a few unmissable achievements in the Misc section. The difficulty achievements will unlock as you complete Act 3. If completed on a higher difficulty, all lower difficulty achievements will be unlocked at the same time.

Operation Quarterback
Complete Act 1


Operation Archangel
Complete Act 2


Left No Man Behind
Complete the game on Casual


My Rifle Is My Best Friend
Complete the game on Medium


Harvester Of Sorrow
Complete the game on Expert

Worlds Apart
Take a successful shot from a distance of 1 km or greater


I don't remember exactly where to get this, but it's the most achieved achievement, so you should get it early on. Regardless, your rifle scope displays the current target distance.

Psst, Don't Look Behind You
Kill 15 enemies with a stealth kill


A stealth kill is done by getting close to an unaware enemy and pressing the required button to perform it. It displays the required button in-game, but it's E by default on PC.

Distance Matters Not
Kill 20 enemies from a distance of 600m or greater


When you scope in on a target, the distance will be displayed on the screen. You should get this during a single playthrough. There are a number of distant snipers you must shoot in the campaign.

Fisher King
Kill 25 enemies using thermal or night vision goggles


You learn how to use thermal vision during the first mission. I believe the default button is V on PC.

Say Hello To My Little Friend
Kill 30 enemies using your sidearm


You will be given a sidearm on every mission alongside your primary sniper rifle. It's pretty easy to get kills with it, especially at close range.

Kill With Still
Kill 50 enemies while holding your breath


While scoped in on an enemy, you can hold shift (by default) to hold your breath. This holds your scope steady and on lower difficulties, slows down time to allow easier shots.

Death's Scythe
Take 50 headshots in single player.


Should get this within a single play through of the campaign. On lower difficulties, a guide circle will slowly appear showing you where to aim to hit your target. Use this to help get this achievement if you're having trouble.

Laugh This Off
In the final duel with Maddox, kill him with the first shot ("No Loose Ends")


This is the very last thing you do in the campaign. After killing the mortar crews and elite snipers, you'll have to take out Maddox himself. He appears at one of three windows pretty much straight across from your position. You have to quickly shoot him before he ducks back out of sight.

If you miss your shot on the first attempt, pause the game and restart the last checkpoint. You go right back to shooting at Maddox and have the chance to get the achievement again.

Hazardous Materials
Kill three or more enemies by shooting a grenade


I didn't put this in the kill achievements section as I believe there's a specific place to get it. During the first mission, you'll be told to find a grenade to shoot at. There is a group of two people standing around with a third coming to join them. The third has a grenade, so wait for him to join the other two before shooting at it.

Nerves Of Steel
Achieve 100% accuracy in any single player mission


Pretty simple if you pick your shots carefully. Give it a go on Casual difficulty to have an easier time with the additional aiming guide and slow motion while holding breath.

Bullets Can't Hurt Me
Complete any act without using medkits


Pretty easy on the casual difficulty. The first act should be the easiest to complete. Just make sure you don't accidentally press the medkit button (Q by default). If you're having serious trouble with it, you should be able to unbound the button in the settings. An act consists of multiple missions and Act 1 has three missions.
Also, nothing here saying you can't die during missions.

I Never Bleed
Complete the Campaign using max one medkit per mission


On casual, you should be okay to go without any medkits at all for most, if not all of the campaign. You can go for this at the same time as "Bullets Can't Hurt Me".

Karma Chameleon
Retrieve your weapon without being detected ("Burning Bridges")


At the start of the mission, you lose your primary weapon during the cutscene. If you follow your spotter's instruction as you progress, you should be okay. You may be able to load the last checkpoint if you get spotted and repeat the nearest area again.

Guardian Angel
Rescue the friendly ("From Out Of Nowhere")


You should get this naturally during the specified mission in Act 2.

Nemesis
Kill Maddox once and for all ("No Loose Ends")


You'll get this one at the end of the game when you get the shot on Maddox.

Balls of Steel
Complete any single player mission without using med kits, but taking damage


You should get this while going for "I Never Bleed". If you don't take any damage throughout the game on your first attempt, you're probably too godlike to be playing games.

Solid Sandman
Kill all enemies in the beach houses without causing an alarm ("Leave No Man Behind")


This is during the third mission in the first act. Doing this on Casual will give you the easiest time. You should be able to load the last checkpoint if you or a body get spotted.

Back To School
Kill all enemy snipers in Sniper Alley without being hit ("Ghosts of Sarajevo")


This one isn't too hard as long as you be careful and don't peek the snipers for too long at once. It's easier on Casual as you can mark enemies and have the additional aim guide.

Hangman's Knot
Kill Major Vladic ("And Justice For All")


You'll actually get this achievement as part of the story during the specified mission.

NOTE: This game, being as old as it is, has a pretty dead multiplayer scene. However, you should be able to create your own server with a friend and go for some of these. You should be able to find someone else on the internet also going for them, so ask around the steam forums or some of the 100% groups.

World Warrior
Participate in any multiplayer match


Pretty simple. Just load into a multiplayer game.

Unhappy Camper
Kill 10 opponents in multiplayer using your sidearm


Shouldn't take too long if you're just grinding with someone. Otherwise, definitely easier with a highly populated server.

Bloodstreak
Kill 25 opponents in multiplayer


Pure Ownage
Kill 50 opponents in multiplayer


The Sniper Elite
Kill 100 opponents in multiplayer


Just a bit of a grindy one. Unlocks over multiple games, of course.

Kill That F**ing Band!
Take 25 headshots in multiplayer


Blood and Tears
Take 50 headshots in multiplayer


Another easy one if you're farming for them, just get other players to stand really close before taking your shot.

Wrath Child
Win 10 Team Deathmatch rounds


Death Dealer
Win 25 Team Deathmatch rounds


Each round is based off the game timer and the winner is highest points by the end of the game.

U Can't Touch This
Win 5 rounds in a row in Team Deathmatch


Absolutely easier with a friend. Just take turns winning the 5 consecutive rounds.

Everything But The Kitchen Sink
In one multiplayer round kill an opponent with a sniper rifle, sidearm and melee attack


Again, super easy with a friend. E is the default for performing a melee attack.

Poking the Bush
Collect all secrets in Act 1


Keepsakes Of War
Collect all secrets in Act 2


In Search of Enlightenment
Collect all secrets in Act 3

3 Missions
Communication Breakdown [4 Secrets]
From Out of Nowhere [4 Secrets]
Leave No Man Behind [4 Secrets]

At pretty much the very start of the mission, the first people you are told to kill are in or near a small shed.
After taking both enemies out, enter the shed from the right.
The secret is just past the generator on the right.

After sneaking under a small pier, you'll climb up onto it and kill an enemy just outside a building.
While in the building behind that enemy, turn to the right.
The secret is on a box just under the stairs.

Later on you'll detonate some C4 to lure some enemies away. Right after you detonate, you walk up this hill.
Instead of following Diaz up, go into the house on the right and immediately go up the stairs on the left wall.
The secret is in the left corner behind an unaware enemy sniper.

You'll eventually reach a church right near the end of the mission. As soon as you enter the front door, but before you walk through the next door, turn to the right.
Take those stairs up the balcony above.
The secret is sitting on the balcony railing.

After coming out of the caves into the first over watch area, you'll see a wooden structure to your right.
Go up the small ramp and you'll see a chair and some other clutter.
The secret is sitting on one of the boxes on the left.

When you enter the first compound personally, you'll see a house positioned in pretty much the center.
Enter the house and you'll immediately see stairs.
Go to either side and you'll find the secret underneath the stairs.

Another two or so compounds after, you'll find a large pool to the right of the objective path.
Go past the pool and enter the lone building nearby.
After entering, the secret is located on the right, past the stairs.

Once you enter this structure with spiral stairs going up to the right, go left instead.
Enter the first opening on the right.
The secret is in this room on a box against the left wall.

Soon after starting the mission, you'll come across a house on the beach.
After clearing it out, before moving on to your next objective, move past it to the far side.
On the ground just behind a rock on your right, near the water, is the first secret of this mission.

After jumping into the water, then clearing out the house, you'll swim up to it.
Climb into the house and head up the stairs.
You will see the secret pretty much right away on a pile of boxes.

Eventually you'll be told to get into a cable car to continue.
Before you do, enter the control room on the right.
The secret is sitting on a table near the door you entered.

After the cable car ride, exit out the right hand side.
Sticking to the right, take the stairs down to a doorway.
Upon entering, you'll see the secret on a table on the far side of the room.

3 Missions
Operation Archangel [4 Secrets]
And Justice For All [4 Secrets]
Ghost of Sarajevo [4 Secrets]

At the very start of the mission you will come across this building.
When the enemy inside has been taken care of, go inside.
The secret can be found on a desk on the right, under a small window.

A little later, you'll come up to an old train station with red and white columns.
Enter the station on the left side of the center platform.
You'll find the secret sitting on a bench where the train on the left reaches.

In the large area with the multiple AA guns, destroy the first gun and move on.
There is a destroyed train near the second gun (the gun that gets destroyed by the air strike).
Enter the train through the door closest to the objective and find the secret right at the end.

When you enter the underground station, turn left towards the objective.
The secret is on a small table across from the place in which you hide.
You either have to be quick, before the enemies come around the corner, or just accept your fate and be killed. (Collectibles stay when you die)

Inside the first building, after taking out a few enemies, follow Maddox and Merinov upstairs.
They will pause on one floor saying they need to go up another.
The secret can be found on this floor by following the left wall on a small desk.

This one is in the area where Merinov pulls his stunt.
When he's done, save him then clear the rest of the enemies.
Go into the first building on the right, and get the secret from the shelves.

This one is after the underground carpark area, where you had to cut the power.
When you get outside, go to the far right near a cage wall.
The secret is in this area in front of a dumpster.

At the end of the mission the objective is to take photographs.
Go right at the top of the stairs instead.
The secret is on a table behind a medkit.

The first secret is in the room where you get your rifle back.
When you walk into the room, you are pretty much facing the rifle.
Turn left from there and you'll see the secret on a low table in the center of the room.

This one is in the entrance of the library building as part of sniper alley.
In the library entrance, look at the right hand side just before the furthest bookcase.
The secret is on a low table, surrounded by a couple of chairs.

After getting through the park, you'll enter a wrecked building.
When you come across an enemy with their back to you, take them out and turn to the right.
You'll see the secret on a desk next to a couple of medkits.

Late in the mission you enter a large mall structure.
Head up the stairs on the right as required, but go left when you reach the top.
The secret is sitting on a low coffee table near some couches, behind a glass screen.

4 Missions
Knife in the Dark [4 Secrets]
Burning Bridges [4 Secrets]
Bad Karma [4 Secrets]
No Loose Ends [4 Secrets]

In the first open area, stick to the right side.
Keep an eye out for the furthest small structure with wooden stairs on the right side and climb it.
From there, turn around toward where you came from and get to the secret by crossing the wooden beams.

When you get to an area where you climb down a ladder, you'll be told to get out of sight of a vehicle.
Get to the building at the highest point opposite the ladder.
The secret is on a table in the upstairs room of the house.

After exiting a cave you'll come to this area with left and right stairs.
Head up both sets of stairs on the right and enter the doorway up there.
At the far corner of the room will be the secret sitting on a cabinet.

When you reach the broadcast building, stop before going up the stone steps.
To the right of this area is a wooden structure with stairs leading up to it.
Climb the stairs and you'll find the secret in one corner of the room, sitting on a small crate.

After crossing the bridge with the spotlight watching it, you'll walk along a wooden structure with enemies around a campfire to your left.
Just before you leave the structure and move past the cave column, turn to the left.
The secret is inside a basket between a wooden pole and the cave column, after facing the enemies around the fire.

A little later, there'll be an enemy that walks out into your path and are told to wait him out.
When he walks away (or you just take him out) follow him back into where he came from.
In the resulting room, look on the right side to see the secret on a crate.

After completing an over watch objective, you'll have to go down these stairs.
At the bottom of both flights, look on the slightly elevated piece of stone floor.
The secret is on a low crate on this elevated area.

Before picking up the explosive, go past them into the back area.
As you enter this area, look to the left and go up the stairs.
At the top of the stairs, do pretty much a 180 turn, enter the compact area and find the secret on the table in there.

After hiding in the bushes with Diaz to take out the two enemies, look for a statue through an archway on the right side.
Go through the arch towards the statue.
Look just behind the statue on its left side.

As you come across an over watch objective, you should be able to see a statue out the broken wall.
Facing the statue outside, go left past the stairs you came up.
The secret is on the ground in the corner.

When you reach a large, open area, clear out the enemies and snipers.
Go towards the large building pretty much opposite where you entered the area.
The secret is on the ground in front of this building, obscured by a wall near some stairs.

After you leave a cave and hide from a vehicle, you'll follow it up the road.
Find the large bell-like object on the left as you enter the area.
Crawl underneath it and look to the left as you leave the structure.

After running through the smoke thrown by Diaz, you'll head up some stairs.
On the platform with Diaz you'll be told to shoot a gasoline canister to distract enemies.
Look on the left wall immediately at the entrance of the area and find the secret sitting on a barrel.

Shortly after you'll go up a path that splits into two.
The objective wants you to go through the arch on the right, but go left here instead.
The secret is sitting on the doorstep of the large building.

Right after being attacked by an enemy in a cutscene and saved by Diaz, you should see an overturned bench in front of you.
Turn to the right from that bench to find a small walled area.
The secret is on a bench on the right side of that area.

Right after taking a ladder up with Diaz, you'll be back outside.
There will be a large fallen column in front of you as you go up a small flight of stairs.
Follow this column to the left and find the secret on a barrel.