Mako Power Plant
 

Items: Potion x3, Phoenix Down, Restore materia, Assault Gun
Monsters: Guard Scorpion (boss)
 

After you get off the train and fight the two soldiers, check their bodies. You should find 2 potions. Also, you need to run so hold the cancel button (originally is X). Head north and fight more enemies. You'll receive potions for defeating them. Find your teammates (Biggs, Wedges, and Jessie) and then name yourself to Barrett. Now follow your teammates and follow yourself through some doors. Talk to some of the teammates in order to get through the doors. Then talk to Jessie in order to get in the elevator and push the arrow to activate the elevator.

Work yourself down the stairs and head through the door on the left. When she stops, look for a potion (it's hard to see because of the steam) and then climb down a ladder and over some pipes. Look for a save point (it a large light object with in the shape of a Question mark) and save. Walk down the walkway and you'll find yourself in a different room with the bomb. Note: Make sure your healed. Pick up the materia near the bomb and a alarm will turn on. A boss will come out and you have to fight it.

Boss: Guard Scorpion...... LV: ?? HP: 800 MP: 0
The Guard Scorpion is a robot. Robots are weak against thunder. So use Cloud's blot magic. Barrett should be attacking. If the Scorpion casts Search Scope, you wont hurt, it will just tell it who's attacking next. When the boss is defeated, it will shake and Cloud will warn Barrett. From this point onward, don't attack it. It'll heal and/or block your moves. If you try not to hurt it,  the boss responds with a damaging laser ray shot from his tail. Attack it a few more times after it stops shaking. It's die after that. After it's dead, you'll get a Assault Gun for Barrett.

You now have ten minutes to get out of it before the reactor blows up. Run yourself back up and get your teammates to help you get out. Also, help Jessie get her foot out. You'll need her. Make sure to equip Barrett's gun so it wont take you too long to fight.