To get the opportunity to fight the Sky Spirit at the end of the game, the order of chosen dialogue options must be 2-1-4-2-1.
This boss fight is broken up into to parts much like when you fight the great earth spirit. Remember to set you’re undead helper to berserk mode and try as best as you can to conserve health for the second part, you may need it!
You will want to leave the undead warrior in berserk mode for the second part of this battle as well. The whole time will most likely be spent dodging the sky spirits breath attack and blue floor tail attack. When the boss goes airborne, you can perform an air attack, or just fire a projectile at it and it will fall back to the ground allowing the undead warrior to resume his attack. Remember, this is easier to achieve on the lowest difficulty! Good luck!
I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.