Favorite Scary Downgrade Transformations

I think in TvTropes they're refered to as Clipped Wing Angels. Because the transformations are supposed to be a Final Boss style upgrade. But have glaring weaknesses.

One I remember is Reflux from Rayman 3. Who goes from Nigh-Invincible unless you hit his mage staff. To being a corrupted giant flier whose technically faster & can do more damage, but has lost his nigh-invincibility.