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