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