The Lighthouse is used as a training area, most of the Trainers inside use Water- or Flying-type Pokémon. An Eletric-type will keep your opponent at bay.
There are no Pokémon in this dark tower, only a series of relatively easy trainers. Some of them can be avoided, but don't miss the Gentlemen, since they carry a ton of cash. The only trick here is to fall through the pit above one of the staircases on Floor 4; that's the way to get to the central staircase that will take you to the top floor. But make sure to head up to the dead end on Floor 5 anyway to get the Rare Candy and TM 34 (Swagger).
Make sure that you've healed your Pokémon before you try to reach the top of the Lighthouse. At the top, you'll find Jasmine, who is nursing a sick Ampharos, Amphy, that usually powers the Lighthouse. Jasmine won't leave the sick animal to battle or get medicine, so she won't be willing to accept any challenges. It's up to you to head over the water to Cianwood City to get the medicine it needs at a drugstore. If you don't pick up the medicine, you won't get to battle in the Gym.