Here is the full list of all 32 Beastmancer achievements.
Train at the small island during the Prologue
Finish the game by destroying the Portal.
Capture at least one of every monster in a single game.
Find every recipe in a single game.
Learn every spell in a single game.
Win 100 battles.
Reach level 20 with your character.
Reach level 20 with a common monster.
Reach level 20 with a rare monster.
Reach level 20 with a unique monster.
Win 20 battles.
Win a battle using only the 4 Testudo variants.
Win a battle using only the 3 Noctua variants.
Finish the game with the permadeath option enabled.
Capture all six legendary monsters.
Spare all your rivals in a single game.
Defeat Nero outside the Arena.
Defeat 1st Warlock Myr
Defeat Lady Aurelia
Defeat the White Lioness
Defeat the Hooded Man
Defeat Diomedes
Defeat Circe
Depetrify all petrified monsters in the islands.
Defeat Alexius
Defeat Marius
Finish the game with your starter monster alive.
Defeat Memnon
Become lord of the Erimune islands.
Use the Cursed Grimoire to summon a powerful monster.
Defeat the Wild Walker
Defeat Paulina.