Instructions
Use your "Refine" skill on the Adamantoise Card that you pick up throughout the game. One of the minigames in "Final Fantasy VIII" involves a trading card game, similar to real-life games like "Magic: The Gathering." As you play against people in the game you get to keep certain cards. When you get the Adamantoise Card, refine it into a turtle shell.
Use your "Mug" skill on the Armadodo when you randomly encounter this enemy throughout the game. As you walk from town to town in "Final Fantasy VIII" you will randomly encounter enemies. The "Mug" command allows you to steal items that they are carrying. Continue to mug the various Armadodos that you encounter until you pick up a turtle shell.
Fight and kill the Armadodos throughout the game. If you have not yet earned the "Mug" command you can still pick up a turtle shell from this creature through regular battles. Fight and kill an Armadodo and see if the turtle shell was among the items it was carrying. Repeat this process until you find the weapon you seek.