To have a constant 10+ hour fight? In an open world? No thanks.

However, let's say you have a fight split in five parts. When you finish one part you have to return another day to do the next. Manage to finish all five in a week and you get an extra bonus (besides the loot from each part).

Every part could introduce new mechanics and even be in new areas (lore wise you could be chasing down a beast).
The final battle could be a top a mountain where it is cornered.

This I could accept. Would give incentive to play and something to do between grinding.