Go north and take the left path. Go up the path and left into the stone building. There is a Save Point here and a Magic Source in the chest. Leave the building and go east across the bridge. Go down the stairs and right under the arches to get the Aurora Armlet in the chest. Now return to the crossroads and go up the middle path. Keep going north until you reach a spiky building. Go left then right to enter it. Go all the way up to get the Comet materia here. Go all the way back to the crossroads and take the right path. Go into the first shell house and up the ladder to get the Guard Source form the chest. Now go north and right into the other shell house. Get the Elixer from the chest on the bottom floor and go up the ladder. Approach the beds and choose to rest.
When Cloud wakes up, go along the beds and check behind the last one to pick up an Enemy Skills materia. Go back to the middle path and go into the spiky building. The fish will have gone, so go down the stairs here. Go down the long set of stairs. Go left out of the building and save at the Save Point. Equip Earth, Titan, Comet, Kjata and Bahamut materia, as well as anything else good. Head down the stairs and across the pillars to get to Aeris. Walk towards her and talk to her. Cloud will draw his sword and talk to her again. Press O three times. Watch the very cool FMV where Sephiroth appears. Cloud will get all sad at Aeris’s death and then Sephiroth will summon another Jenova for you to fight to Aeris’s music…
Use Earth, Titan, Comet, Kjata and Bahamut as they work particularly well against this boss. Physical attacks do quite a bit of damage. But be careful as Jenova has a nasty attack called Aqua Lung, which does about 1,500 HP of damage, but do try to stay alive as it is a good enemy skill to learn. Make use of your Cure/All materia. Also Haste and Barrier are a good idea as well. You’ll win a Wizard Bracelet.