The handicap you get before the last (8th) round has the biggest effect on your BP by far. If you can get All Materia Broken before that battle, it'll give 10000BP if you win; otherwise you could try Magic Materia Broken for 7000, that's good too. If you're strong enough not to rely on materia much then you can go for that, and you get BP pretty quickly. Try to avoid weapon broken or items stealed at all costs.