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