ultima weapon is very easy. you just need a few basic spells and Hp around 1000-1700.
he either attacks one person and takes off a large chunk of their HP or does minimal damage to all of your party members. these can all be countered by minwus cure spell. my minwus cure spell was about level 12. it was a long battle because of not being able to do giant amounts of damage but quite an easy one. BTW is SoR only on DoS? overall ultima weapons difficulty is much overrated