Well, anyway, best strategy, short mode
Lorelei: Zapdos will decimize most of this group. with thunder and thunderbolt or whatever electric attack you happen to have
The second guy(Rock/Fighting): Vaporeon, Venusaur, Articuno, Zapdos. Zapdos i threw in for the Drill Peck attack, which should take care of any fighting types. The others you can take using Surf, Solar Beam, Blizzard/Ice Beam, whatever. Be sure not to get hit by a Rock attack when using Articuno, though.
Agatha: Marowak only, or Ground-type atks. They're all dual-poison. bring awakenings, though. If you had a fighting-type I'd suggest Foresight, then a solid fighting atk, as Gengars are pretty low in defense.
Lance: Articuno, Vaporeon(with Ice types). Zapdos for the Gyarados. Watch out for the Dragonite.
Gary: Zapdos for Pidgeot. I'm not sure about how you should take care of Alakazam. Dark types and ghost work, but I don't think you might have any of these atks on your pokemon. Rhydon: Strong Ice or water. Gyarados: Zapdos. Exeggutor: Zapdos, Moltres, Articuno (Drill peck/Fire/Ice). Charizard: Zapdos at a strong rock-type attack, like Rock Slide. You can learn it in that cave (the cave where there's Mankeys Geodudes, Onixes, etc. Or, the cave between Lavender town and Vermilion.). I still advise you against going, as Gary's P/Kmon are +lv.60.