Here is the full list of all 28 Princess Battles achievements.
You have learned about Gerald's past.
You have married a commoner.
You have put a rival to good use.
You have traded cards on the black market.
You have won a different kind of card game.
You have eloped with a commoner.
You have experienced life as a commoner.
You have defended your territory.
You have kissed a commoner.
You have fought a general's daughter.
You have learned that somebody has been keeping track of your progress.
You have married a prince.
You have improved your relationship.
You have refused a marriage proposal twice.
You have seen somebody's true form.
You have fought a visitor from the North.
You have kept secrets from Aurelia.
You have hidden your true identity.
You have opened a school.
You have ascended the throne.
You have become a good ruler.
You have won against Olivia.
You have unlocked the Freeplay Mode!
You have learned about Lena's motives.
You have faced a demon princess and won.
You have fought a bounty hunter.
You have learned that somebody has been gathering information about you.
You have learned about Olivia's past.