Follower of the Cross am I?
This sounds like either Fujin or Raijin, both of whom may be called 'Followers of the Cross'.
If the Ruby you seek I may deny
Does Ruby refer to the Carbuncle summon? Or does it refer to a different summon?
Best be quick in case you fly!
This boss could easily have you 'fly' by an ability.
Ultimate end is your reward when
Sounds link the 'ultimate end' is either the Eden summon, which I highly doubt, or access to Ultimecia who is also the final boss of the game.
I reach for the sky!
This boss reaches for the sky.
Just throwing it out there.