If you are at the Lighthouse that means you still have a little more trainers coming up before your next badge (Chuck). Chuck has Fighting/Water Pokemon so I would suggest you train your Sunkern instead of getting rid of it. You will going on to a water route with some trainers which is good training. You may also want to do what myrrh25 said and get a leaf stone at the PokeAthlon. Just try not to get rid of Mega Drain/Solarbeam. As for the Fighting Type of your next Gym, I recommend you weaken them down with Pidgeotto if you can, and finish them off with Feraligatr. Then when you head back to face your sixth gym (after you healed Amphy) you will be up against Steel/Ground/Thunder Pokemon. Your newly trained Sunkern or Feraligatr should be able to take care of Steelix. But for the other 2 Magnemites, avoid using Feraligatr and Pidgeotto. Try tracking down a fire Pokemon. I believe there are some Growlithes in Route 36/37. Or if you caught Sudowoodo, he can be useful against the Magnemites too.