This is a story related achievement and cannot be missed.
*** Spoiler - click to reveal ***
After you save the village in the beginning of the game, you're asked to explore the fjords to kill the Jokull that's threatening your people. Just exit the village by the upper left corner portal to reach the fjords, where the Jokull awaits you in a cave.
He's not hard to beat, just avoid his freezing attacks, step in the red circles when you're too "cold" and spam your attack on him.
When you defeat it, the achievement is yours.