I don't know how much you all are into the Creature Creator part of X-2, but i thought it might be helpful to those of you who are interested, to post how to recruit Ultima Weapon for your own use.
what you need to do is find certain fiends around Spira and train them until their entire Fiend Tale is unlocked. the requisite fiends are: Daeva, Aka Manah, Aeshma, Jahi, Tomb, Dolmen, and Monolith, in that order.
Daeva will change into Aka Manah during its ending, which will change into Aeshma during its ending.
Aeshma will perish in its own ending, and you will automatically recruit Jahi.
after getting Jahi's ending, you will be told, in cryptic fashion, to capture a Tomb. if you're in Chapter 3, this is your stopping point.
unlocking Tomb's ending will allow you to capture Dolmen in Chapter 5.
to get Monolith, you must have done 2 things prior: unlocked Dolmen's ending and have visited the Farplane Abyss once.
once Monolith is captured, unlock its ending, view it, and Ultima Weapon will be automatically recruited.
hope this helps! =^-^=
Iron Courage. need I say more?