Looking at your team, I'd say that you will have problems with:
- Clair (Ice Beam, Thunderbolt)
- Misty (Thunderbolt)
- Sabrina (Bug Buzz doesn't do much damage since it is Special, and they are Specially defensive.)
- Blue's Gyarados (Thunderbolt)
- Will's Xatu and Slowbro
- Karen (except Vileplume and Umbreon)
- Red's Blastoise and Lapras
hmm... You need to alter your team a little bit. Camerupt is a bit useless here as it overlaps types with Gastrodon and Typhlosion. I suppose Forretress is useless in-game, but it is up to you whether you should use it or not. You can give Ice Beam to Gastrodon.
Typhlosion, Gastrodon and Forretress are fixed for your team. I would like to make a lot of changes in your team. Here you go:
- Leave Yanmega and get another Flyer such as Fearow. You need Fly HM to move from one place to another easily. Useful moves for it: Return, Fly, Drill Peck and any other move. You can catch a Spearow very early.
- Options for 4th 'mon:
- Heracross: Megahorn, Brick Break, Earthquake, Rock Slide / Stone Edge / Rock Tomb. It is extremely useful for Will, Karen, Pryce, Sabrina (I doubt though), and Red's Lapras. Headbutt trees in Ilex Forest.
- Weavile: Night Slash, Ice Shard, Brick Break, Poison Jab / X Scissor. Plays same role as Heracross, but is a little frail. Sneasel is obtained in the route 28.
- Primeape: Cross Chop, Assurance, Rock TMs, Thrash. Sabrina wrecks it, but other than that it is good.
- Ampharos is definitely your sixth 'mon. It kills Water types very easily, and is a great team mate of Typhlosion. Thunderbolt, Thunder wave, Signal Beam, Light Screen. It also supports your team (Heracross and Weavile) against Sabrina with Light Screen.
Good moveset for the rest
- Lava Plume / Flamethrower
- Sunny Day
- Solar Beam
- Focus Blast / Hyper Beam / Eruption
- Whirlpool -> Waterfall
- Mud Bomb / Earthquake
- Gyro Ball
- Bug Bite / Toxic Spikes
Hope this helps :)