-Equip a Dragon Armlet and Water Ring on one person in your party. This will make Blue Light and Blue Flame do half the damage and Aqualung cures that person.
-Equip the Aurora Armlet (which you can find nearby) to your hardest hitter. Un-equip every Materia you have on that person. When you start the battle, use as many summons as possible before he casts some sort of barrier. That barrier will deflect magic that you cast on him. After that, just keep using attacks or D. Blows on him. By now your other teammates will be dead. Do not revive them. Notice that everything Jenova does to hit you will just revive your health. Keep doing Attacks or D. Blows until you win. Note: Once the screen reads "Jonova-LIFE's skill power is used up", you can revive your partners because he will not attack.