Things You'll Need
Instructions
Play through the game until you take control of the Buggy vehicle just after completing the "Gold Saucer" area.
Ride the Buggy back the way you came all the way to the city of Junon. Go into Junon while still in the Buggy, which will transport your party and your vehicle to the eastern continent via ship.
Go back to the cave near the Mythril Mines that is on the other side of the river you were unable to cross previously. Use the Buggy to cross the river and go inside the cave.
Examine the sleeping man inside. He will tell you the number of times that your party has fought enemies in combat. As long as the final two digits of the number of battles you have fought in are the same, the man will give you some spare Mythril. If the last two digits are not the same, simply go outside and fight in some random battles.
Return to the main continent and find the house near Gold Saucer. Go inside and speak with the weapon merchant who lives there, who will ask you for some Mythril. Give him the Mythril to receive an item called "Great Gospel," which is Aeris' best limit break.