Here is the full list of all 20 ARENA an Age of Barbarians story achievements.
Kill Kan-Dread the Destroyer!
Perform a 10-hit combo with any enemy!
Kill that damn goblin!
Win a fight without getting hit!
Win a fight without parry!
Perform a 20-hit combo with any enemy!
Dismember 100 enemies!
Do a Special Move 30 times!
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