If this new art is of 'Ultima Weapon' instead of a different style of Bahamut, they could make it work like VII's Ultima Weapon fight (which they somewhat incorporated into XI with Sandworm, Dark Ixion and the VNM Yilbegan) where it's a world spawn (minus VNM version) but it travels through zones and while you can attack it, it'll run away from you (Ixion) unless you use a specific item to keep it in place and claim it.

Much like in VII Ultima Weapon will fight then run away after x damage and go to a random location (or straight to where the ancient forest is iirc, but I hate that game so I may have forgotten) and be the final showdown, so whoever gets the ultimate claim at it's 'resting spot' will be the victorious party, unless the alliance system is implemented at that point in time.