Try leveling your team within 5 levels of the Elite 4's pokemon and you should be fine with some recovery items. Having an electric type will be good against Gyarados/Blastoise/Pidgeot, and a dark type (if you can get one) will be good against Alakazam/Exeggutor. Rhydon is super weak to water and grass. For the most part, you should be able to one-hit Gyarados and Rhydon, and Pidgeot won't be much of a threat. Alakazam and maybe Arcanine could be a challenge, but it'll be fine if you use something like Light Screen or have a pokemon with high s. defense.