Ultimate series should'nt be one fight of 20 min, but instead 3 fights with a difficulty growing (Twintania : 7 min, Nael : 9 min, Bahamut : 11min).
The total difficulty shouldn't be changed overall.
It would permit to players to go in more easier, because the fights would be less longer, there would be at least one reward by fight (because some people need rewards more than challenge itself to advance. With a fight of 20 min, you never see the end).
The sentiment of progression would be better at my sense.