Get in close to him

You move in close, knocking aside his blade to allow you to move in close. Now that you're close, his long ranged great-sword isn't as powerful as he has no room to swing it, allowing you to bash his side with your mace. Your mace wallops into his chest with a powerful smash, breaking ribs. Karstuk is sent stumbling backwards, at which point you bash his sword arm with another strike, knocking it away. Another swing knocks away Karstuk's legs, and he's left collapsing to his knees.

With that, Karstuk is left kneeling in front of you, looking up at you.

"Finish it, human," he spits, looking up at you as he realizes he's been defeated.