On a scale of one to ten, UB is a three. The thing about UB is that he has more HP than anything you have seen up to this point (except Nemesis or Penance). So prepare for a long battle.

The best thing to do is to have stats above 125 in all areas except Luck, assuming that you are finding the Luck Spheres difficult to obtain at this time.
*cough* Earth Eater/Greater Sphere *cough*

The first thing to do is to destroy the Head, and the best way to do this is either with Wakka, or someone who knows how to use Ultima. If you are capable of doing 75000 with Ultima or more, then it should also destroy the arms (which protect the main body).

After that, pound away. UB will alternate between Ultima and physical attacks, so you must have balanced stats.

I would not suggest using Aeons at this point in time (their stats are also too low, and to gather the items needed increase their arsenal of attacks is painfully difficult). And you need to try; you don't get Game Over at the Monster Arena, and there is a very handy dandy heal less then 1 second away.

When the arms are dead, UB takes FULL DAMAGE from both Magic and normal attacks. When the Arms are alive, attacks directed at the Body will cause damage as though you had base stats. The Arms have the "Armored" status and cannot be Armor Broken, but they do take full damage form Magic attacks. The Arms regenerate every three of UB's turns. The Head also takes full damage from all attacks at all times.