Ultima Weapon can be easily won if the player equipped Ricard with the Blood Sword in the mainstream game, letting him have the weapon in Soul of Rebirth. Even then, the player needs to keep Ricard alive until he takes up all of the HP from the monster.
Otherwise, it is best to have the party's HP up to a minimum of 2,000 - 2,500 and Minwu have sufficient training in Protect and in Shell spells, for these spells will help reduce the damage from Ultima Weapon's Blaze spell. Haste and Berserk are rather useful - provided the player can keep the Hasted and Berserked character(s) alive.
A chancier strategy, but one that will allow a player to beat Ultima Weapon at relatively low character strength, is to use an Ancient Sword, one of which can be found in the Unknown Cave, to inflict Curse on the monster. It has a low chance of hitting and a low chance of inflicting the status, but Ultima Weapon is not immune to Curse. It is recommended that the wielder of the Ancient Sword also wield another weapon to increase damage output, as the Ancient Sword is not very strong on its own. The Curse Tome can also be obtained as a drop from Mini Satanas found at Unknown Palace. Of course, buffs should be in place and characters should still be trained in Cure spells, as the party will have to survive however long it takes for Curse to connect. If Curse connects, though, Ultima Weapon's attack and defense will drop sharply, making it possible for a low-level party to survive its onslaught long enough to whittle away its impressive HP.