How Do you Beat Giovanni?

That would depend on the game you're playing however if you're playing FireRed and LeafGreen he will have Pokemon such as Onix, Rhyhorn, Kangaskhan, Nidorino, Nidoqueen, Dugtrio and Nidoking and for Pokemon such as Nidorino, Nidoking and Nidioqueen you can use Psychic-type Pokemon in order to beat them, Rhyhorn and Onix you can use Grass-type Pokemon in order to beat them, Dugtrio you can use Flying-types in order to beat it and you can use Fighting-type Pokemon in order to beat Kangaskhan.