Here is the full list of all 50 The Walking Dead Onslaught achievements.
Finish Chapter 2: Strange Circumstances (Any Difficulty)
Kill an Armored Walker
Discover a New Weapon for the Armory
In Scavenger, Get to the Rendezvous Point Without Taking Damage from the Wall
Find a Supply Cache in a Supply Run
Upgrade a Weapon in the Armory
Finish Chapter 1: Desperate Days (Any Difficulty)
Complete a Scavenger Site
Choke 30 Walkers
Construct the Town Hall in Alexandria
Bring a New Survivor to Alexandria
Finish Prologue: What Comes Before and After (Any Difficulty)
Finish the Tutorial
Finish Chapter 3: The Rabbit Hole (Any Difficulty)
Finish Chapter 4: Around the Bend (Any Difficulty)
Finish Chapter 5: Roads Less Traveled (Any Difficulty)
Finish Chapter 6: When Dust Settles (Any Difficulty)
Finish Chapter 7: The Promise (Any Difficulty)
Finish Chapter 1: Desperate Days (Veteran)
Finish Chapter 2: Strange Circumstances (Veteran)
Finish Chapter 3: The Rabbit Hole (Veteran)
Finish Chapter 4: Around the Bend (Veteran)
Finish Chapter 5: Roads Less Traveled (Veteran)
Finish Chapter 6: As Dust Settles (Veteran)
Finish Chapter 7: The Promise (Veteran)
Finish all Campaign Chapters (Any Difficulty)
Finish all Campaign Chapters (Veteran)
Replay and Finish Any Chapter from Chapter Select
Kill 5 Walkers With One Explosive Trap
Kill A Walker With Every Weapon in the Game
Kill a Spike Walker
Kill 1,193 Walkers
Bring 10 New Survivors to Alexandria
Reach Maximum Population (25) in Alexandria
Earn All Achievements
Complete all of Rajul's Requisitions and Fully Upgrade the Town Hall
Complete all of Hazel's Requisitions and Fully Upgrade the Stockpile
Complete all of Sydney's Requisitions and Fully Upgrade the Windmill
Complete all of Julia's Requisitions and Fully Upgrade the Forge
Complete all of Oliver's Requisitions and Fully Upgrade the Clinic
Complete all Requisitions
Construct all Buildings in Alexandria
Fully Upgrade all Buildings in Alexandria
Fully Upgrade Any Weapon in the Armory
Fully Upgrade All Weapons in the Armory
Complete Alexandria (Max Population, Buildings Fully Upgraded, Campaign Complete)
Discover all Weapons for the Armory
Unlock all Scavenger Sites
Complete All Scavenger Sites
During Evac, Survive for 3 minutes after the Van Arrives