Here is the full list of all 11 Poly Duels achievements.
Defeat a non-easy AI as Amon
Defeat a non-easy AI as Siegfried
Defeat a non-easy AI as Longdarion
Defeat a non-easy AI as Satoru
Defeat a non-easy AI as Ito
Defeat a non-easy AI as Seiichi
Defeat a non-easy AI as Alyss
Knockdown an enemy
Kill an AI enemy with full HP with one hit
Win against a hard AI without taking damage
Complete the tutorial