Well... I carried out the aforementioned optimisation and then some extra stuff.
Biggest time save was attacking down on horseback, that means you can go full speed throughout. It is also waaayyyy more reliable than slashing from the side once you get the hang of it.
The hardest bit is getting the on-the-move horseback shot on target, I spent about an hour repeating up to that stage (the narrator's voice gets a bit annoying after a while!) until I finally got that.
Time I measured was 1:16:80.
Good luck beating this!