Try running in circles around the outer edge and do not try to force hits onto Dark Fact unless he flies into your running path. I was frustrated as well (on normal) but the fight is fast and quickly over.Fuck Dark Fact. I liked Ys 1 up until the Darm Tower. The tower itself is manageable, despite some absolute bullshit sections, floor 17 with that linear hallway and those asshole scorpion things or the completely nonsensical smash the nondescript pillar to pass through the hallway of death puzzle, but Dark Fact can go fuck himself. That boss is such complete and utter bullshit. Constant barrage of spam that lands where you were standing, the impacts have AoE's so there's more spam and then when you successfully land a hit on Dark Fact, the ground falls away. If you happen to be standing on that section of the floor, you're instantly dead. I keep getting him to half health and dying and, for reasons unknown, can't even use my equipped health potion. That's such a dumb arbitrary restriction.
I like Ys VII and I'm enjoying Ys IV but god damn if Ys 1 isn't trying my patience. I never want to try and play this game on Nightmare.
Humble thanks to gngf123 for the Mystery Box!