Humm i remember having Holy junctioned to status atack and it did 0 points of damage with a single atack, but i usually beat her up with a single limit break by Irvine, Squall or Zell when one of them is selected if you keep using your limit breaks and your characters have good stats then you´ll beat Ultimecia easily believe me Omega Weapon is much more tougher than all of the forms of Ultimecia combined.