If it's 2.0, then you need to first whittle down her defences using low-cost spells/attacks as she'll lose some defence or magic defence whenever hit by an attack of that type. There's a few ways of going about this efficiently:
Fire is a good option as it's low MP cost and also has its own innate stacking defences reduction (1%) that'll make the process faster; Braver and Climhazzard are also good as they have this same effect and with a higher %-reduction (I think it was about 10-15% reduction per use).
Support Materia like Added Cut are also a good choice because they're counted as a separate action, meaning you can cast a spell to lower magic defence and then it'll follow up with a physical attack to reduce her physical defence (or you can attach it to a Command Materia like Deathblow to reduce her physical defence twice in a row).
Another option is to use your newly acquired Comet spell which ignores magic defence and will let you deal decent damage albeit at a higher MP cost; Comet is a good choice for early on until her defences have been reduced enough that you can swap over to conventional elemental magic and attacks. Laser and Summons also ignore magic defence so these can be used as well.
The other side of this is standing up to her attacks; I've made a list of the attacks and their effects below:
Red Light: Non-elemental, magic-attack
Nuclear Pores:
? Element, magic-attack, reduces target's defence stats by 20%
Silence: Like the spell, inflicts Silence
Gas:
? Element, magic-attack, inflicts Death Sentence
Iron Summon: Physical attack, Blunt/Shoot element
W-Iron Summon: Same as Iron Summon but stronger and less accurate
Death: Inflicts KO with a roughly 50% chance, is used once in the fight
[Counter] Meteorain: Shoot element, magic-attack, used at 25% HP.
[Counter] Nemain: Raises defence stats by 25%, grants Wall, used at 50% HP
[Counter]Everyone's Grudge: Deals damage equal to target's kills * 5
The Protect Vest accessory can be handy for the physical attacks as it reduces shoot-element damage. The
? element can be protected against by using the Guard Ring acquired from the chest in the Whirlwind Maze before the boss, or by combining elemental with a non-elemental Materia (I think Command Materia are an option for it). Generally though I think Barrier and MBarrier are the go-tos for this one so I would combine that with one of your furthest-along All Materia.
Some KO protection would be handy but it isn't essential to have it on everyone; the Safety Bit provides that, or Added Effect + Destruct, and the enemy skill Deathforce (which can only cover one target at a time mind; if the caster uses it again on a different target, then the previous target of Deathforce loses its protection). If someone is inflicted with Death Sentence and doesn't have KO protection, then Dispel/Holy Mirror can sort it out, or you can use a Black Veil item (or the Deathforce skill) if you still have it to sort them out. Otherwise, keep the Life spell handy to bring someone up who gets hit by the Death cast or has their Death Sentence timer clock out or failing that have remaining characters take their turn together for phoenix down + healing.
For silence there's option like Seal + Added Effect, Fairy Ring, or the Rune Armlet, but having an Echo Drop on hand would also suffice if you want to focus on guarding against other things instead.
One hidden option is the accessory Relic Ring; what this does is essentially turn the character into a 'zombie' with the following effects: maxed defence/magic-defence, absorb poison, gravity, and
?, and block the KO, sleep, poison, confu, silence, frog/mini, petrify, dual, death sentence, and blind statuses. However as a trade-off, it makes the character take damage from restorative spells and also blocks the Regen status so you would want to heal this character using Poison/Gravity attacks instead of conventional healing spells. This accessory was sitting in a chest in the area after the Sleeping Forest, just before exiting onto the world map to reach the Forgotten City (where Water Ring originally sits).
Finally, have Sense active on Jenova Death so you can keep track of her HP; once she hits 10,000HP or below she'll use a one-time counter-attack of Meteorain so you want to be nice and healthy before hitting her past that point or failing that have all 3 characters alive to spread the damage a bit. You should also have Debarrier handy for the 20,000HP mark to get rid of her Wall from Nemain. The counter-attack at 30,000 HP, everyone's grudge, will depend on who it is casted on; if that character has unlocked their Lv.3 limits then it may hit a bit harder as they'll have more kills but on the whole it tends not to be overly dangerous unless they ground out their Limits very early on and are actively in the party as a damage-dealer.
