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