If you have Yuffie, you have Throw materia. Level it up to get Coin and throw 99,990 gil at Biza. He'll take 9999 damage to all unguarded areas until parts start to die, then he'll start taking 9999 everywhere else as long as you keep hurling that amount of money. Once you run out, don't worry. Just remember to get rid of his defenses & the core, and then just attack w/All hitters and Limit Breaks.