How Do I Kill Lost Number in FF7?

Equip Aeris with the Choco/Mog materia before opening the safe. The Choco/Mog summons casts "Stop" on all enemies on top of its physical attack. Set Aeris' "Limit Break" level to Level 1. Her "Seal Evil Limit Break" will also come in handy. Open the safe. Lost Number will attack automatically once you enter the combination.

Attack Lost Number with your most powerful magical and summons attacks, particularly Choco/Mog. Choco/Mog's "Stop" ability has a good chance of paralyzing Lost Number, leaving him open to attack for several turns. Avoid using physical attacks. Lost Number will respond to physical attacks with a powerful counter-attack that is likely to kill the targeted character unless he is paralyzed.

Use all Limit Breaks on Lost Number as they become available. These are generally your characters' most powerful attacks. Aeris' "Seal Evil" is particularly useful, as it stands a good chance of casting "Silence" on Lost Number, preventing him from casting his powerful "Ice 2" spell on the party.

Continue attacking Lost Number. He will morph into a new form once his HP falls below 3,500. The form he takes depends on what kind of attack was used to bring his HP level below that point. He will turn purple and use only physical attacks from that point on if it was a physical attack. He will turn red and use only magic if it was a magical attack.

Use your strongest physical attacks if Lost Number turns red. He is weak against physical attacks but strong against magic in this form. Use your strongest magic attacks, including summons, if he turns purple. He is weak against magic in this form. You will get the Odin materia and access to the locked room in the basement once he is defeated.