Here is the full list of all 20 Days of Doom achievements.
Start your first run.
Save a Raider in an event.
Defeat King and Gizzard.
Defeat Baron Sledge.
Have 6 members in your squad at the same time.
Bring every character to Sanctuary alive at least once.
Fully upgrade the settlement.
Unlock Merchant building.
Unlock Pyro.
Unlock Thrasher.
Unlock Boomstick.
Unlock Colossus.
Reach Sanctuary.
Fully equip a character. (All rune and item slots)
Reach a boss node with only a single character left alive.
Reach level 6 with a character.
Lose your whole squad during a run.
Run out of fuel during a run.
Dispatch 500 enemies.
Finish a run without losing a single squad member.