When you go up to the coffin in the room, you´ll meet a man named Vincent. You have to destroy the army going up the hill. It's important to note here that if you play the piano anyways and complete the first line, you'll receive Tifa's hidden savings, which amounts to an amazing 1 gil. Also make sure each character has a powerful summon materia. As you enter it Cait Sith will give you inside information on the Shinra. Aeris' limit break should keep you healthy enough so just keep pummeling him.
Cloud, saddened by the party´s loss, vows to never forgive Sephiroth for what he has done. Cloud made a promise with Tifa long ago to help her in a time of need. In combat A bridge will appear in the room below if you do it. The path is linear but long, and there quite a lot of enemyss on the way so don´t go trough too fast. Surface when you hit the big tunnel type area and you´ll be near a waterfall. You have to ride a chocobo past the snake.
Besides, it'll count as just winning the regular battle, so you'll also get the Huge Materia. As he does, the Weapons awake from their slumber be left with a cool sequence. Head north until you find an underwater cave. Hop on the Highwind again, as you move north you will see that the crater has been opened and you can head for Sephiroth. Time your movements through the barriers and you will run into Sephiroth. Cloud says he need to go to Sector 7. Keep in mind that if some characters get knocked out he can still pick up the last member of your party and its game over.
Continue on the path and go north. Most is explained in the manual. Launch the submarine from the harbor and head near the south, but watch out for Emerald Weapon, which is one of the Weapons siming around down here. You will come across a reporter who is actually Yuffie in disguise. With it you can explore the ocean floor. They are called Huge Materia and are needed to make the Master Materia. Also, Sephiroth shows the picture that was taken that day.
When you are done, Misson Complete will appear and you can now use the sub on the World Map. Talk to the old man and he will tell you how to climb the cliff. You will see another Sequence. Talk trough everything, and you will get the item you need. You see Barret and the rest of Avalanche engaged in the defense of the Pillar. Take a nap and you´ll notice your cell door is open when you wake up.
You really can´t do much here, but talk to the people and learn about the area. Make sure you keep everyone alive if possible, its just that much harder if he decides to grab you. When you catch it you will jump out to their train. This is a very hard boss and you won´t be able to take it out now. Should the timer reach zero, the Huge Materia is lowered into the ground. He quickly learns that there is more to being a hero than physical strength and fame. Go up towards the other part of the graveyard.
Equiping items that resist fire is a good idea for the next boss. Approach the face in the wall at the end od the cave and you´ll encounter Gi Nattak. Head up and you will find Aeris in prayer. Getting the items laying on top of the trains and in the barrels are beneficial. Board the Airship and go to Condor Fort. You will learn about the Shinra´s space program and how something terribly went wrong so it never entered space. You will learn more about Aeris' wish to protect the planet and the White Materia.
There are hints telling you what the combination is hidden within the rooms. Now search the jar in the front of Turtle´s Paradise and you will have her surrounded. Palmer is only a nuisance standing between you and the Tiny Bronco, so quickly kick his big butt to get him out of the way. Failing two challenges will result in missing out on the Blue and Red Huge Materias. When you win this fight, yiu will get a Peace Ring. Line yourself up to where all of the diggers are pointed and search and you will find the Lunar Harp. Go into the booth and hit the switch and keep following the track to the city.