Despite being the third one you discover, Hearthome City Gym in Pokémon Brilliant Diamond and Shining Pearl can only be explored once you've helped Cynthia's grandmother in Celestic Town. If you've taken that goal down, head to the gym to face Fantina and her collection of Ghost-type Pokémon - arguably the hardest you'll ever fight.
How to solve the fifth gymnasium puzzle
Getting to Fantina is easy enough, as you will only have to solve basic math equations to take the doors leading to her. However, without having to read a single equation, simply take the doors in order from right to left and you will be taken to the Fantina without fighting another trainer.
Which Pokémon does Fantina use?
Like the previous Gym Leaders, Fantina will only have three Pokémon in her group, namely Drifblim (Lv. 32), Gengar (Lv. 34), and Mismagius (Lv. 36). As you can see, this is a steep step up from the last gym. Therefore, you may want to enter some random encounters and level up your party before rushing into this battle.
Additionally, we recommend that trainers bring a full range of heals with them as they battle Gengar and Mismagius. They will constantly try to confuse your Pokémon with their Confuse Ray ability and then use their most powerful move when your Pokémon can't attack.
The best Pokémon to use in the fifth gym
As we've warned, this battle is going to be harder than anything before, it's simply because Ghost-types don't have many weaknesses. The remakes use the classic Generation IV strength and weakness system, so Ghost will only be vulnerable to Dark and other types of Ghosts. As these types are quite rare to come across, there are a few ways you can be successful without one.
With Headstocking Drifblim out first, make sure you have a Rock-type to lead your team. Drifblim is also a Flying-type Pokémon, so having a respectable monster like Rhyhorn in your arsenal makes this a quick match. You can grab a Rhyhorn for yourself from one of the many Grand Underground Hideaways. As a second option, there are several rock-type TMs that can be purchased from the Stonevelo Department Stores and applied to many Fighting and Ground-type Pokémon.
The fastest way to take out Gengar will be to use Psychic and Ground-type moves. The Pokémon is also Poison-type, which makes these types of attacks super effective and should beat this opponent in just two hits. We recommend that you catch a Psyduck or golduck from the Great Marsh Safari of Pastoria City. As shown above, Golduck is capable of learning harmful moves based on both of these types.
Since Mismagius is purely Ghost-type, it's best to have Pokémon that can learn Ghost-type and Dark-type moves on your team. The most common that can do this is machop, which can be discovered in any area between Route 208 and Route 211. Pokémon's most effective moves against Ghosts are Rematch and Knock Off, the latter of which can be learned at level 12.
Related: How to evolve Machoke into Machamp in Pokémon Brilliant Diamond and Brilliant Pearl