Here is the full list of all 50 The Walking Dead Onslaught achievements.
Fully Upgrade all Buildings in Alexandria
Complete Alexandria (Max Population, Buildings Fully Upgraded, Campaign Complete)
Finish Chapter 7: The Promise (Any Difficulty)
Finish Chapter 7: The Promise (Veteran)
Finish all Campaign Chapters (Any Difficulty)
Discover all Weapons for the Armory
Reach Maximum Population (25) in Alexandria
Complete All Scavenger Sites
Complete all of Oliver's Requisitions and Fully Upgrade the Clinic
Finish Chapter 6: When Dust Settles (Any Difficulty)
Finish Chapter 6: As Dust Settles (Veteran)
Finish Chapter 5: Roads Less Traveled (Any Difficulty)
Finish Chapter 5: Roads Less Traveled (Veteran)
Choke 30 Walkers
Construct all Buildings in Alexandria
Unlock all Scavenger Sites
Finish Chapter 4: Around the Bend (Any Difficulty)
Finish Chapter 4: Around the Bend (Veteran)
Kill 5 Walkers With One Explosive Trap
Kill 1,193 Walkers
Finish Chapter 3: The Rabbit Hole (Any Difficulty)
Finish Chapter 3: The Rabbit Hole (Veteran)
Kill a Spike Walker
Bring 10 New Survivors to Alexandria
Complete all of Rajul's Requisitions and Fully Upgrade the Town Hall
Find a Supply Cache in a Supply Run
Finish Chapter 2: Strange Circumstances (Any Difficulty)
Finish Chapter 2: Strange Circumstances (Veteran)
Kill an Armored Walker
Fully Upgrade Any Weapon in the Armory
During Evac, Survive for 3 minutes after the Van Arrives
Discover a New Weapon for the Armory
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
Finish Prologue: What Comes Before and After (Any Difficulty)
Finish the Tutorial
Finish Chapter 1: Desperate Days (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 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 Requisitions
Fully Upgrade All Weapons in the Armory