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