Here is the full list of all 20 ARENA an Age of Barbarians story achievements.
Dismember 100 enemies!
Kill Kan-Dread the Destroyer!
Kill that damn goblin!
Do a Special Move 30 times!
Perform a 20-hit combo with any enemy!
Perform a 10-hit combo with any enemy!
Win a fight without parry!
Win a fight without getting hit!
Perform 30 Counterattacks!
Kill the giant serpent!
Rescue Xodan from the torture chamber!
Defeat the sorcerer Ghoor!
End the game as Rahaan
End the game as Sion
End the game as Mombata
End the game as Sheyna
End the game as Ayla
End the game as Xula
End the game as the Deathbringer
End the game as She-Red