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