Here is the full list of all 50 The Walking Dead Onslaught achievements.
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)
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
Construct all Buildings in Alexandria
Complete all of Oliver's Requisitions and Fully Upgrade the Clinic
Unlock all Scavenger Sites
Finish Chapter 6: When Dust Settles (Any Difficulty)
Finish Chapter 5: Roads Less Traveled (Any Difficulty)
Kill 1,193 Walkers
Fully Upgrade Any Weapon in the Armory
Kill a Spike Walker
Finish Chapter 4: Around the Bend (Any Difficulty)
Kill 5 Walkers With One Explosive Trap
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)
Find a Supply Cache in a Supply Run
During Evac, Survive for 3 minutes after the Van Arrives
Finish Chapter 2: Strange Circumstances (Any Difficulty)
Kill an Armored Walker
Choke 30 Walkers
Upgrade a Weapon in the Armory
Finish Chapter 1: Desperate Days (Any Difficulty)
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
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 (Veteran)
Replay and Finish Any Chapter from Chapter Select
Kill A Walker With Every Weapon in the Game
Earn All 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 Requisitions
Fully Upgrade All Weapons in the Armory
Complete All Scavenger Sites