Lost Number
This Boss can be tough, but here's an easy solution. Add Aeris to your party and set herLimit Break to Level 1. She can use Seal Evil against Lost Number and paralyze it for most of the battle. While it's paralyzed have Cloud and your other party member pummel it with strong spells. You should be able to knock it down to half size before it recovers from the paralysis.
If the purple side (attack) dies, the red side (magic) will be casting spells against the party. If the red side dies, the purple side will assault your team with physical attacks. Fight the red side with your own physical attacks. The red side of Lost Number is very resistant to magic; fight the purple side with magic. Summon "Choco/Mog" and you might put paralyze the monster again; if not, use Aeris's Seal Evil as soon as her Limit Break is ready. For winning this battle, you get the Cosmo Memory, Red XIII's Ultimate Limit Break. Don't forget to get what you came for, get the "Key to the Basement" from inside the safe; which does seem like much, but it has a better use than you would believe. Don't forget to grab the awesome Summon "Odin" on the floor.
NOTE: Which side lives and dies depends upon the last attack before the transformation. If the last hit is a Spell, then you'll face the red side; if the last attack is a physical blow then you'll face the purple side.
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