After you explore Anistar City and take a look at the giant, ancient sundial, go to the Anistar Gym.
The Anistar Gym is a Psychic-type Gym. Psychic pokémon are weak against Bug, Ghost, and Dark moves.
In addition, Dark-type pokémon are immune to Psychic attacks, so you might want to have a lot of Dark-type pokémon in your party.
However, one of the pokémon here has Fighting-type attacks, which is strong against Dark, so you might want to have at least one non-Dark pokémon with you.
To make the gym appear, step on the symbol on the carpet in the first room.
There is a dead-end to the south, so start by going north, then west at the fork. To the north you'll encounter Psychic Paschal, who has a level 46 Exeggutor. It is very weak to Bug moves, and somewhat weak to Flying, Poison, Ghost, Fire, Ice, and Dark moves. It has a Grass attack in addition to a Normal and Psychic attack.
Go north from there to find a warp tile. Step on it, then go east, then north at the fork in the path. Continue east. You'll reach another warp tile. There is nothing to the north, so just step on the warp tile.
Go south, then west at the fork in the path. Use the warp tile at the end, then go south, and west at the fork in the path. Continue south to find another warp tile. Use it, then go south. Keep following the southward path, ignoring the paths to the right. Eventually, you'll pass Psychic Harry, who has a level 46 Medicham. It's weak to Flying, Ghost, and Fairy moves. It has Fighting attacks.
Go south, then east at the fork (there's nothing to the west). Eventually you reach another warp tile. Use it, then go north and use the warp tile there. From there, go north. If you use the east path, you can avoid the battle with Psychic Arthur, who has a level 46 Reuniclus. It's weak to Bug, Ghost, and Dark attacks. It uses Normal and Psychic attacks.
Now go west, then south to reach another warp tile. Use it.
For an optional battle, go north and you'll encounter Hex Maniac Arachna. She has a level 46 Jynx. It's weak to Rock, Bug, Ghost, Steel, Fire, and Dark moves. It uses Normal, Ice, and Fighting attacks. Go south after fighting her.
Go west from the warp tile. You can take the north path to avoid Hex Maniac Melanie, who has a level 46 Gardevoir. It is weak to Poison, Ghost, and Steel moves. It uses Psychic attacks.
Keep going west. At the far west end, there is a warp tile. Step on it. There is a dead-end to the east, so go north. You'll find the gym leader, Olympia.