As he displayed an innate talent at an early age, I figure he's got some pretty healthy base stats. I've given him a chain hauberk, which goes down to his knees and has full sleeves. Here's a pic of Arn from the movie:
He has a chain hood, long chain sleeves, and his hauberk goes down to his knees. I've given him leather (AC8) on his shins, otherwise he is AC5. He wields the light two handed sword, which is meant to represent a real two handed sword. The heavy two handed sword it the mythical dragon slaying variant.
Ok, so here's Arn's sheet:
I've held off giving him Feats, as that's another matter entirely and depends on the system you are using.
He has a CR of 50, a CP of 19, and a dex bonus of 10 to add to the mix. I've added a box for Combat Maneuver Rating (CMR). This is his CR, minus his CP, plus his dex bonus. It is representative of his overall strengths and weaknesses, and is a great indicator of initiative rating. This number can be used for a wide variety of things.
Edit: the CMR dex bonus rating is d100, not d20. I'll fix the character sheet soon.
At a CR of 50, Arn fights equivalent to a 10th level fighter. As such, he gets a second attack at +25 each round.
A great strategy for Arn would be to parry with his first attack. A parry subtracts the CR from the opponents CR, potentially forcing his opponent to roll a 01-05 to hit. He then gets a second free attack in the round at +25.