If you're not ready to enter the crater, get out and prepare. When you're ready, land in the crater like you would land on the ground. Make sure you're stocked up on Ethers and Phoenix Downs, most of the enemies here can easily cause lots of damage or status effects including Sadness, Mini, Poison, Instant Death, and so on. Also try to stock up on items like Elixirs and Sources here. If you have enough time and patience, you can max out several stats. In the first room, be sure to pick up the Save Crystal. This is the most important item in the Cave since there are no Save Points to fall back on here except the 1 you create. Did I mention that it's a long way down? Ok, into the cave.

Enemies

Your biggest threat in the first few rooms is a Dark Dragon you may encounter. It has 14,000 HP and can counter your Ultima spells with Ultima. However, it knows Dragon Force and can be manipulated, so use it to learn the skill. Use your best other attacks to defeat this menace. Also, you can encounter Gargoyles that cast Lv 5 Death as their Final Attack, so be careful. Your other threat is the Sadness status caused by the Parasites. And as a last mention, the Master Tonberrys in the Cave have 80,000 HP or so, and can kill in one hit with knife. Once you get down the enemies will change.

The Two Paths

If you want, you can put a Save Point here, but it should be saved for farther down. Now comes a hard part; you must split your party into 2 groups. Whoever goes with you can be chosen in your party, so choose wisely. Take the left path on the right(You can explore the right paths on your way up.

The Left Path

On the column you start at, you should spot a chest there before the drop. It contains a well needed Mystile. Grab it and continue.

In this section of cracked eggs, grab the Tetra Elemental and go on until you reach the end of the cave. Place your Save Point here if you haven't already placed it somewhere else. Save, and head back the other path.

Next Set of enemies

These enemies on the Left Path have some power if you encounter the right ones. The Malboro makes a reappearance here, so give it a healthy dose of water. The Dragon Zombie If encountered, can easily be killed by curative magic, since after all it is a zombie. One note of caution though. The Dragon Zombie can cast Shadow Flare, which causes lots of damage to any one of your team. Try and learn it though, because it is a kickass move. The other enemies you really shouldn't need my help with, though.

Right Path

Head up the open path. Time your button press when you jump on the platform with the purple Materia on the first screen on it so that it is pressed the moment you hit the platform to pick up the Mega-All Materia. Pick offall the other items. When you reach the top, turn right to reach a room that starts the "third" path.

Enemies on the Right Path

You encounter the Master Tonberry again on this path too, so watch out. You can also encounter a King Behemoth. It can counter most powerful magic with Comet 2, so be careful of that as well. It also attacks with King Teel and Bite, both of which do signifigant damage to your party members.

The Other Right Path

This path allows you to get the Counter and W-Magic Materia. Be careful of Magic Pots, who cannot be harmed until you feed them an Elixir. If you have Elixirs to spare, use them to gain almost 10K Exp, 10K Gil, an 1000 AP. Most other enemies are easily defeated. steal Turbo Ethers from the Armored Golems, though.

Armageddon?

When you're ready, Save at the Save Point, then proceed down the path to select your party. I suggest using the party you have usually used for other major battles. Only the two WEAPONs surpass the difficulty of this. Make sure your party is equipped, then head out.

The Final Leaps

On your way down, you will encounter enemies like Dragon Zombies, the heavily armored Iron Man, or anything else powerful in the cave. When you make your way down to the final platform, prepare for a series of nice tough battles.

Jenova-Synthesis

Beware of the tentacles, which can cause various statuses. Take them out, then focus on the main body with everything you have, pausing to knock the tentacles back down when neccesary. When Jenova starts counting down from 5, heal your party, set up Wall, then unload everything you've got left onto Jenova and pray that you kill it. If you don't, then you will be hit hard by Ultima, Wall or no Wall. If you kill it, you will get 35000 Exp and 2500+ AP. Now get ready to fight Sephiroth, choose your party/ies and prepare yourself.

Bizarro-Sephiroth and Safer Sephiroth

Bizarro-Sephiroth has one hell of a lot of HP. Its core heals it every turn for about 6000 HP, so you need to take out the core to destory the all-important torso. But to estroy the core, your going to have to destroy all other parts first. This is why it's good to have only one party. Once the core is down, take the torso with some normal attacks.

Safer Sephiroth

Safer Sephiroth uses attacks like Flare, Break, Wall, Despell, etc. The offense attacks he has easily do 2000+ damage, so you might want someone as a healer the whole battle. Restore MP, then pummel Sephiroth with your most powerful strikes. Eventually, Sephiroth will go out of Short range, so when he does this, heal up and set Wall. He will then use Super-Nova, which leaves everyone in critical or very close to it. Heal, and repeat. When he is defeated, you'll have one last battle at this guy. I'm pretty sure you can't lose. If you do, then you're absolutely pathetic. Now watch your well-earned victory cinema!!!