This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Ultimate weapons have zero Materia growth, but have eight holders, and
every two holders are linked. When using the 'Morph' materia, the amount
of damage inflicted drops to 1/8th, but with an Ultimate Weapon, the
amount only drops to 1/3rd. There are also special ways to increase the
amount of damage these weapons do:
Cloud Strife's 'Ultima Weapon':
* Kill the Ultima Weapon during Disc 2 or 3 to get the Ultima Weapon.
This weapon does damage based on the (lack of) difference between
Cloud's current HP and maximum HP. In other words, the fuller your
life is, the more damage you do. Note that this isn't similar to
the way the Atma Weapon worked in FF III.
Barret Wallace's 'Missing Score':
* Stick Barett in your party while climbing up the stairs to the Mako
Cannon during the raid on Midgar during Disc 2. If Barett's in your
party, a chest will be visible on one of the landings that contains
his Ultimate Weapon. To empower Barett's weapon, attach materia to
it that have lots of AP (such as mastered materia).
Tifa Lockheart's 'Premium Heart':
* Get the Sector 5 Key and return to Midgar during Disc 2 or 3. Go to
the Wall Market and enter the building labeled "Item". Examine the
computer and you'll get her weapon. During a battle, Tifa's weapon
does more damage the closer she gets to maxing her Limit Break.
If she does a Limit Break and misses one of the slots, the weapon's
power will momentarily increase, then lessen.
Aeris Gainsborough's 'Princess Guard':
* While exploring the Ancient Temple during Disc 1, open the chest in
one of the corridors connected to the clock puzzle room in order to
get the Princess Guard. If her allies are in critical condition
or are killed, the power of her weapon shoots up. (Note: Materia
has normal growth in the Princess Guard - it's the only ultimate
weapon which has materia growth.)
Red XIII's 'Limited Moon':
* Prior to the raid on Midgar City in Disc 2, or during any time in
Disc 3, go to Cosmo Canyon with Red XIII in you party. Bugenhagen
will give him his Ultimate Weapon before dying. The Limited Moon
does more damage depending on how much current MP Red has, just
like how the Ultima Weapon's ability to inflict damage depends on
Cloud's current HP to maximum HP.
Yuffie Kisaragi's 'Conformer':
* Her Ultimate Weapon is found in one of the chests in the wrecked
Shinra plane that's lying on the sea floor. When fighting enemies,
if they are of a level higher than Yuffie's, she does more damage
Cait Sith's 'HP Shout':
* Found in the back row of lockers on Floor 64 of the Shinra HQ,
during the raid on Midgar City in Disc 2. Just like the Ultima
Weapon, Cait does more damage depending on how close his
current HP is to his maximum HP.
Vincent Valentine's 'Death Penalty':
* Put Vincent in your party and visit Lucresia during Disc 2; she's in
the Waterfall Cavern on the Western Continent and you can get there
using a Green Chocobo or the submarine. You should go here as soon
as you're able to. Then, visit Lucresia again before you go off to
kill Hojo, and she'll give Vincent the Death Penalty and his Extreme
Limit Manual, Chaos. The Death Penalty's power raises in proportion
to how many enemies Vincent has killed since joining your group.
Cid Highwind's 'Venus Gospel':
* After the Rocket Huge Materia Quest, return to Rocket Town. Talk to
the old man who gave Cloud the Yoshiyuki. Chat with him repeatedly
and he'll produce the Venus Gospel for Cid. Like Red's Limited Moon,
the amount of damage this weapon does depends on how close Cid's
current MP is to his maximum MP.