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