This is due to his spin attacks, armor/power debuff freeze cloud, and his near-death mode when he shoots homing icicles. You will want to stay as far away from this boss on the other side of the area.
So you will generally only be able to hit it if it is looking in your direction after one of its spin attacks. This boss has minor weak spots in its knee joints on its legs, but its maximum DPS weak spot is its face, which is a narrow orange/yellow slit on the front of its head. You will only need to beat it on Normal, but the achievement will also unlock if you beat it on Extreme. This boss is story-related and can't be missed.
