Final Fantasy VII/Finale

Here your party reunites, and it's time to tackle the final boss. At the top of the stairway leading to the center of the planet, there's still one more chance to first grab all of the cool trinkets the other folks have collected, get your gear in order, and pick your team before heading down.

The descent
The stairway consists of fights with some really annoying enemies. Floating eyes (Allemagne), big knights with bigger swords (Iron Man), and skeletal dragons (Dragon Zombie), will plague you the whole way down. The eyes and dragons are fairly easy, though the latter's Pandora's Box Enemy Skill is quite dangerous. Be very careful of the knights, which are very tough. When fighting Iron Men and Dragon Zombies, it's best to treat each fight with equal seriousness as if it were a boss fight; start the fight with Big Guard and throw some big summons at it. Bring a lot of either with you so you can heal yourself afterwards and regain your MP.

At the bottom of the stairs, you find the last iteration of the Jenova monsters.

Sephiroth
After the fight, you end at what may be the center of the planet. It is rather hard to say where you are at the moment, but you do know that Holy is close by, as is Sephiroth. For the following fight, you will get to split your party into one, two or three groups, depending on how well you got here.

Finally, Bizarro Sephiroth falls, and you head right into the next battle.

That was the last fight in the game, the next one is just a nice cinematic showdown between Cloud and his nemesis, Sephiroth.

The end
Sephiroth is destroyed in a rather unexplained way. It is quite difficult to tell for sure, though, what really happened at the end. If the end seems weird, there is plenty of fan analysis to be found. In the end, Holy tries to destroy the Meteor, the Lifestream tries to stop everything, and it all fades to white. After the credits, you see Red XIII return to Midgar 500 years later.