Progress through the story until Chapter 4
Go to Goug Machine City
Visit the shop and trigger the scene about the missing stone
Go to Zarghidas Trade City
Find the Flower Girl event
Choose to help her and buy the flowers
Return to Goug Machine City
Watch the scene that opens the way to the Deep Dungeon route
Go to the Deep Dungeon entrance area
Trigger the battle at Nelveska Temple
Defeat the boss there
Obtain the Materia Blade
Keep Cloud in your party and equip the Materia Blade to him to use his abilities