Demandred is older than almost any human being around in today's age. Even before the War, LTT and Belal at least discovered fencing as a sport. If they trained hard, treating it as a sport, for decades that no post-Breaking sword wielder (only men wield swords- AS don't use weapons, Aiel don't use swords, Sea Folk see their men as the warriors, damane don't need swords, male channelers die young and even then Rand learning and insisting the BT learnt to fight even though they could channel was pretty unique and many idiotically questioned him for it) could possibly train for, they would gain a skill hard to match for a mortal human, which could then quickly be adapted for actual killing in the years of the War of the Shadow. There may also have been sword fighting in the Collapse, which lasted a Century, and Demandred has evidently been honing his skills on release from the Bore