Things You'll Need
Instructions
Acquire the Magical Lamp. This is a missable item that has to be obtained during the mission briefing for Squall’s first SEED mission. Speak to Cid twice during the briefing to obtain the Magical Lamp.
Prepare your characters. Make sure you have three characters in your active party, and that all three of them have the “draw” command equipped. This can be done from the “junction” option under the main menu. You will also want to make sure that you junction the strongest spell you have to your hp, as a large HP total will help quite a bit for this fight. Equipping one character with the magic ability will also help when it comes to healing.
Bring up your item menu. Select the Magic Lamp. This will summon Diablos.
Draw “demi” and cast it on Diablos. This will reduce his HP by ¼ every time you cast it.
Keep an eye on your HP. If your characters get low, heal them as quickly as possible, using potions or magic. Diablos can cast various spells that will deal quite a bit of damage to your whole party at once.
Keep casting demi on Diablos until it stops doing large amounts of damage. Remember that when you cast it, it takes ¼ of his current HP. This means you can figure out exactly how many HP Diablos has left.
Attack Diablos to finish him off. Using this method, it shouldn’t take more than 3 or 4 minutes to take him down from full. You can never kill him from casting demi, since it only takes a percentage of his life. Once you get him down so far with demi, however, one physical attack is all you will need to defeat him.