Here is the full list of all 17 Ancient Enemy achievements.
Beat the game in Relaxed mode
Beat the game in Normal mode
Beat the game in Nightmare mode
Own all active ability cards
Beat chapter 10
Earn 36 stars in a single chapter
Make a combo of 50 or higher
Beat chapter 6
Save 4000 power without spending it
Gain 2500 power in a single chapter
Gain 1500 power in a single chapter
Fill all nine available wyrm card slots
Use three ability cards in a single hand
Beat chapter 3
Earn 20 stars in a single chapter
Make a combo of 20 or higher
Make your first combo