Winter Bird is one of those annoying final bosses that have an area-of-effect 1-hit-K.O. attacks.
If you get him down to half of his health he shifts into a gryph form and regains all of his health, plus gains a higher defence.

He also deals massive amounts of ice damage, is resistant to spells and is basically completely over-powered to the point where you scream and rage and curse your computer or console of choice to the 9th circle of hell, until you just give up at the end.

But you give it another try later because he’s just so amusingly suave and chill (no pun intended) in his unskippable cutscene.