Watch out for the tsunami. To unlock this achievement you are must defeat the Order Knight Niedzielan and destroy the Cerulean Chain without letting any of the tidal waves hit either Capell or any of the other characters you have with you. First attack Niedzielan until he is stunned (knelt down), then you can attack the chain. Repeat this three times until you kill Niedzielan, rather than stunning him, and destroy the chain in one hit, rather than damaging it.