Here is the full list of all 16 Banners of Ruin achievements.
Win a combat with a combined Party Vitality of 3 or less
Play 20 cards in a single turn
Defeat the Garrison Commander
Reach 10 Stamina during combat
Reach a party of 6 characters
Apply 30 Poison in a single combat
Forfeit a character card choice
Win 50 combats
Banish 10 cards in a single combat
Reach 40 or more cards in your deck
Have 30 or more Charge during combat
Defeat the Elite Brutes
Defeat the Elite Bruisers
Hire a character at the tavern
Complete the Tutorial
Defeat 3 enemies in a single turn