When you first go to the Chocobo ranch you have to catch a chocobo to avoid the Midgarsorm. Well, I figured out a way to get the powerful Beta spell before the second disc. You need the Enemy skill materia, a Fire materia, and any elemental materia equipped on one person.

MAKE SURE THE LAST TWO ARE ON YOUR ARMOR!

If you keep this character healed until Midgarsorm casts Beta you SHOULD be able to survive the spell. Then just finish him off. This is tricky to pull off but if done correctly you will be set for a LONG time.