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
Make a combo of 50 or higher
Own all active ability cards
Beat chapter 10
Fill all nine available wyrm card slots
Beat chapter 6
Gain 2500 power in a single chapter
Earn 36 stars in a single chapter
Save 4000 power without spending it
Use three ability cards in a single hand
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