Difficult. Easily more difficult than Outlander S-Ending. Old Knight Armor has like 20x the health on Hard Mode on same pieces, and the Old God you have to fight took like 20 turns.
The 3 year old S-Ending Dark Mage run is substantially easier than this one since it does not incorporate the Hard Mode changes above.
Updated version done with no Empty Scrolls and with a side of gabagool: https://www.youtube.com/watch?v=VvqGwuBQYXw