The Darknut is a very common late game enemy and is the miniboss guarding the Bow in the Tower of the Gods.
Walk up to the Darknut and immediately do three consecutive quickspins and a regular sword slash on its armor. Now parry the Darknut when he gives you the option to do so and if you get an overhead parry do a stab to knock off the Darknut's armor from behind. Now it should only take a single quickspin attack to finish him off. The reason this works is because the damage done to the Darknut's armor gets applied to the Darknut's regular health if you knock off the armor from behind and don't destroy it.
All late game Darknuts can be defeated by using a combination of ice arrows and the skull hammer. Ideally you'll do an overhead parry on the Darnut and can then target him to easily shoot him with an ice arrow. A Darknut is not frozen for very long, so right after shooting it you'll need to pull out the skull hammer and do a jumpslash to shatter the darknut before it regains control of itself. if you happen to get a parry around the darknut and knock off its body armor, shoot the darknut without targeting it as targeting will shoot the ice arrow at the darknut's head.