Jon was a prodigy with a sword in hand and practised relentlessly from a young age. Harry Potter was a prodigy in combat and killing. With barely two years of serious training, he could resist one of the most dangerous Dark Lords and even kill him by surprise when he had dropped his guard down. After surviving for 300 years and as the sole survivor of a hundred-year-long total war, his instincts and skill in combat were virtually unsurpassed. It was no surprise that it only took him three days to become rather dangerous with a heavy greatsword.