Find: 1 Luck Source & whatever items that your friends found.

Get the Luck Source from the chest here. Talk to all your friends and get the items they found. If you wish, leave and take the other paths. From this point you can still go all the way back and leave the crater.

Now run off the bottom of the screen if you're ready. Form a party, then go. Follow the platforms down to the bottom, fighting the enemies the whole way down. When you get to the bottom, you fight Jenova-SYNTHESIS.

Use Big Guard, then your strongest physical and magic attacks. Ignore the arms (parts B and C), they will be killed anyway if you use Ultima. After Jenova-SYNTHESIS A's HP gets low, she counts down. Kill her before she reaches 0 or she will cast Ultima.

Use Big Guard, then do an all out attack on Sephiroth. Nail him with everything you've got. You can switch parties every time you kill a body part, but I don't think you'll need to. Beware, because he can cure himself. Also, don't attack part C, you can't damage it until all the other parts die. Parts B, D, and E are easy to kill. Part A can revive the others.

Big Guard again, then hit him with your strongest Summons, magic, Enemy Skills, Coin Toss, Limit Breaks, and everything else in your inventory. Slow works on him very rarely, so try it where it's with another attack, like Summon Hades. He is a fairly easy boss, compared to the two hidden Weapons.

Beware of his Supernova Summon-type attack. It takes your HP down to Near-Death, although it looks really cool. This is not blockable, and sometimes confusion and silence. Also, he has a Heartless Angel attack, which reduces your party’s entire HP to 1. He can use MBarrier, Shadow Flare, Break, Deen (a fire attack), Pale Horse (which is like Bad Breath), and his physical attacks do around 5000 HP of damage.