Final parry felt a little sketchy, only because he idly jogged, rather than performing a weapon art attack from wakeup. But I'm sure it's something to dial in on my end. Make sure to bait to the back of the room before the first parry, wait several seconds after following projectiles, and leave enough room to run across the room to the other side.
This is a fairly minimalist strat in the sense of levels and ease of materials. The fire gem is right on the way in early game. And the 30 END, 52 DEX loadout is easily attainable, even when purchasing all materials for a +10 dagger, and with Cinder and Midir still alive. A deep infusion on the dagger would be a little better, and a sharp infusion with any decent buff certainly better. The strat does not rely on having Flynn's ring either. If using Flynn's ring or substantially higher DEX, it would be a good idea to alter the combo on Halflight at the start.