Jenova's only unique ability for this round is a countdown at the end of the fight (if it takes that long). At the end of the countdown Jenova will cast Ultima and then die, so as long as you can survive it you're good. Make use of your best abilities because she has 60,000 HP, it can take a good while to get through if you haven't completed the side quests.

First of all, before the fight begins, you will be given the option to create two parties, and during the fight you will be prompted to switch over to the other party. This is pointless. Unless both parties have good Materia, and leveled up Limit Breaks, there's just no reason to switch to them. Whichever party you defeat this boss with is the party that moves on to the next Sephiroth form.

This boss has 5 targets for you to take down, the most important being the center piece, which has the ability to revive the other parts. It has to die ASAP. Try to use moves that will hit all of the body parts at once, such as Ultima, Contain, and Summons.

This is the only boss that actually changes depending on how well leveled your characters are up to this point. He'll be tough no matter how far you've progressed through the game. This is Sephiroths last true form.

The One-Winged Angel will cast wall on himself, which can be removed with DeBarrier if you have it. Otherwise this is going to be one long fight. His primary attack, Pale Horse, hits for a ton of damage. Your healing character should save their turn to heal immediately after this move, or the battle could get out of hand fast.

Sephiroth, from time to time, will also use his most famous move, Super Nova. Take a break when he casts this move, go make a sandwich, finish your laundry, and he might be finished casting it by the time you get back. It hits all party members for a fair bit of damage. He will fly out of range, high above your party, before he casts it (which means only characters with a ranged attack can hit him).

Remember that all your items and moves are fair game at this point. There's no point in saving Turbo Ethers, Elixirs or Megalixers, as once you've defeated this boss the game is over.