Agatha is the Ghost-type master of the Elite Four. But looking at her team, she might as well be the Poison-type master. To be fair, there aren’t a lot of Ghosts to choose from, so you can’t really blame her. Anyway, Ground-types are very welcome. Plus Psychic-types for anything that’s not Gengar.
Her first Pokémon is Arbok, an evolved Ekans. This venomous snake can use Poison Jab for STAB, Crunch to damage Psychics, and Glare to cause Paralyze. Ideally you want to one-shot with a super-effective Pokémon before it cripples your lead with Glare.
Next, she may sent out Gengar–but be careful as she has two. Both are weak to Ground, Psychic, Ghost and Dark. The Level 53 one can use Shadow Ball and Sludge Bomb for STAB, and Will-O-Wisp to cause Burn. Unlike Arbok, Gengar is fast, so it may use Will-O-Wisp before you. If possible, field a Dark-type special attacker.
Golbat is a foe you’ve faced time and time again. This giant bat is weak to Electric, Rock, Ice and Psychic. Agatha’s Golbat can use Air Slash for STAB, Crunch to punish Psychics, and Quick Attack to try and go first. Any of the aforementioned Types are great, but switch out your Psychic if it has low HP.
Weezing is an evolved Koffing and a pure Poison-type like Arbok. This one can use Sludge Bomb for STAB, Shadow Ball to damage Psychics, and Thunderbolt to surprise Flying and Water-types. Ground-types are ideal and Psychic-types should be fine if they are healthy.
Finally, Agatha has another Gengar. This Level 54 one can use Shadow Ball and Sludge Bomb for STAB like the first one. But instead of burning you, it can use Dazzling Gleam to hurt Fighting, Dragon and Dark-types. As such, stick with Ground unless you think your Psychic or Dark-type can outlast it.