Who dares disturb my sleep?
Diablos is an optional boss in Final Fantasy VIII. The player summons Diablos into battle by using the Magical Lamp, and fights it to acquire it as a Guardian Force. If careless, the player can lose the chance to obtain the Diablos GF by selling the Magical Lamp.
Just after graduating as SeeD, the party is sent out on a mission. Headmaster Cid briefs the party on the Timber operation and afterward, gives Squall the Magical Lamp. The lamp's description advises saving before use, foreshadowing a possibly difficult fight.
Upon using the lamp in the inventory, the current party is transported to an alternate dimension where Diablos is fought. Diablos can be fought with either Squall's or Laguna's party, as the Magical Lamp may be used at any point in the game.
Though Diablos is a Gravity-based enemy, he himself is not invulnerable to Gravity, and his attacks are not all Gravity damage. Diablos will cast Curaga on a party member that casts Demi from their spell stock, instead of just draw-casting it. Diablos also counter-attacks spells cast against him with Demi.
Diablos's Gravija attack will reduce the party's current HP by 75%—although said attack cannot defeat the party, it leaves party members at an HP level in which they become susceptible to death from his strong physical attacks. Diablos can be affected by Blind (with only 50% resistance against it) and Slow; while blinded, he still has a chance, albeit small, to hit.
Diablos's level is the average level of the party that fights him. If fighting Diablos when he is level 30 or below, Demi and Cura magic can be drawn from him; if level 30 or above, Holy and Flare magic can be drawn, as well as Demi, and Curaga replaces Cura.
The easiest strategy to defeat Diablos as soon as Magical Lamp is available is to have different characters junction Quezacotl and Siren for the Mag-J ability. When the battle starts, the party should inflict Diablos with Blind and Slow and cast Protect to mitigate damage from physical attacks.
Even a low level party should have at least some Protect and Blind spells in stock, drawn from Granaldo and Raldo in the Balamb Garden Training Center during graduation. There was also a Blind draw point at the Communications Tower in Dollet. Slow is available from level 20+ Caterchipillars in Balamb forests. Blind and Slow spells can also come up in Selphie's Slot.)
The party members should have the Item command and hold a stock of Phoenix Downs. Once Diablos is blinded and slowed, the party should stock Demi and then cast it from their stock to deplete Diablos's HP (even more effective if the player has stocked Double from the Elvoret), while causing him to counter with Curaga, effectively removing the need to heal.
When Diablos is blinded, he is unlikely to hit, and his physical attack only targets one character. Therefore, the player may opt to not heal from his gravity attacks seeing as they cannot KO, and use Limit Breaks while in critical health to finish the battle quickly. If Diablos's attack happens to connect even when blinded, the others can revive with Phoenix Downs.