Zaan the Scalecaller is the final boss in the dungeon Scalecaller Peak. This is actually a fairly easy achievement to get and can be done on normal.
The shield attack is a melee attack. Stick to ranged weapons and magic from a distance and you will easily unlock. Don’t worry about the poison or ice attacks as they do not impact the achievement, nor does dying assuming it is not from Zaan’s shield attack.
I used a stamblade with a bow and circled the outer edge to stay away from Zaan, then switched to dual wield to quickly take out the ice statues. Once those were clear, retreat and reengage with bow.
Achievement will pop shortly after the end of the battle.