These achievements can be achieved simply by playing the game.
Saving Fergus and Wyatt can be done by replaying the mission.

Gunner
Saved by allies
Happens in the first mission

Fergus Saved
Save fergus
Choose for fergus, this options happens when you meet Deathshed

Wyatt Saved
Save Wyatt
Choose for Wyatt, this options happens when you meet Deathshed

Vive la resistance!
Locate the resistance
Simply play the game for this to happen

Power to the laser
Find the Laserkraftwerk
Simply play the game for this to be found

Hidden in the deep
Locate the underwater stash
Simply play the game for this to happen

London uprising
Defeat the London Monitor
Simply play the game for this to happen

Deliverance
Release friendt
Happens in chapter 15, simply play the game for this to happen

Liberation
Complete the game on any difficulty
Finish the game on any difficulty

Super hero
Complete the game on I AM DEATH INCARNATE.
Finish the game on I AM DEATH INCARNATE! difficulty or on ÜBER and get 2 for 1.

Über hero
Complete the game on ÜBER
Finish the game on ÜBER difficulty

All these achievements are related to getting your stealth perks in the game, if you want to see the progress go into Journal (default J) click on perks and have a look.

Scout I
Unlock Stealth Perk 1
Stealth kill a commander

Knife Throwing
Unlock Stealth Perk 2
5 silent take downs

Knife sheath +
Unlock Stealth Perk 3
5 stealth kills from throw knives

Knife sheath ++
Unlock Stealth Perk 4
20 stealth kills from throw knives

Silent shot
Unlock Stealth Perk 5
10 stealth kills using silenced handgun

Vampire
Unlock Stealth Perk 6
5 silent takedowns with overcharged health

Scout II
Unlock Stealth Perk 7
5 commander stealth kills

Assassin
Unlock Stealth Perk 8
Stealth kill 50 soldiers and 5 kampfhunds

All these achievements are related to getting your Tactical perks in the game, if you want to see the progress go into Journal (default J) click on perks and have a look.

Deadeye
Unlock tactical perk 1
Perform a headshot while aiming using Assault rifle (3) and Handgun (1)

Quick Draw
Unlock tactical perk 2
40 Headshot kills

Quick Regeneration
Unlock tactical perk 3
Overcharge your health to reach 200

Gun Magazine +
Unlock tactical perk 4
15 kills from cover using handgun

Shotgun Magazine +
Unlock tactical perk 5
10 Indirect kills using shotgun shrapnel mode

AR Magazine +
Unlock tactical perk 6
80 Kills from cover using assault rifle (primary fire)

Marksman Magazine +
Unlock tactical perk 7
3 kills using marksman without exiting scope

Quick Reload
Unlock tactical perk 8
100 kills Assault rifle, 50 kills handgun, 25 kills marksman

All these achievements are related to getting your Assault perks in the game, if you want to see the progress go into Journal (default J) click on perks and have a look.

Double Reload
Unlock assault perk 1
10 Dual-wield kills

Endurance I
Unlock assault perk 2
3 kills while sprint-sliding (single or dual wield)

Scavanger
Unlock assault perk 3
3 kills in a row using dual-wield assualt rifles without releasing the fire button

Bullet Feeder
Unlock assault perk 4
20 kills with a mounted turret

Endurance II
Unlock assault perk 5
5 kills within 10 seconds using a detached turrer gun

Autopanzer
Unlock assault perk 6
Empty a fully powered LKW without missing

LKW Battery +
Unlock assault perk 7
5 Indirect kills using LKW reflection upgrade

Dual-wield expert
Unlock assault perk 8
100 Dual-wield kills and 25 LKW kills

All these achievements are related to getting your Demolition perks in the game, if you want to see the progress go into Journal (default J) click on perks and have a look.

Throwback
Unlock Demolition Perk 1
Kill 2 enemies with one grenade

Grenade Pouch +
Unlock Demolition Perk 2
10 grenade kills

Grenade Pouch ++
Unlock Demolition Perk 3
Kill a commander with a grenade

Bullseye
Unlock Demolition Perk 4
KIll 5 mechanical enemies with a grenade

Rocket Magazine +
Unlock Demolition Perk 5
Kill 5 enemies within 10 seconds using dual-wield rocket launchers

Vaporize
Unlock Demolition Perk 6
5 indirect kills by dropped enemy grenades

Sentinel
Unlock Demolition Perk 7
Kill 5 enemies with thrown back grenades

Hardened
Unlock Demolition Perk 8
50 grenade or rocket kills

These achievements can be achieved when you collect all items.
Or incase of the enigma you can also find guides for those.

All that glitters
Collect 25 gold items
Find 25 gold items to get this.

Heart of gold
Collect all gold items
Find all gold items to get this.

The lives of others
Collect all letters
Find all letters to get this

Secrets revealed I
Solve the first Enigma code
Collect all first enigma codes

Secrets revealed II
Solve the second Enigma code
Collect all second enigma codes

Secrets revealed III
Solve the third Enigma code
Collect all third enigma codes

Secrets revealed IV
Solve the fourth Enigma code
Collect all fourth enigma codes