In Diamond, Pearl and Platinum, Bertha is the second member of the Elite Four challenged in Sinnoh. Her team is comprised of Ground-type Pokémon, although some are part Water. With this said, it is best to avoid using Electric-type Pokémon against her, as they will not last long in the battle. The best alternative is to have either a Water or most recommended Grass-type Pokémon because she has part-water type Pokémon, such as Whiscash or Quagsire, when fighting her. The strongest Pokémon on her team is Hippowdon. Her Pokémon levels range in the 50's, so it is wise to be prepared with Potions, Revives, etc., and leveled up properly. In Platinum, her Pokémon will get stronger on following rematches.
"Well, well. You're quite the adorable Trainer, but you've also got a spine. Ahaha! I'm Bertha. I have a preference for Ground-type Pokémon. Well, would you show this old lady how much you've learned?"
"Dear child, don't assume for an instant that you've won."
"Well! Dear child, I must say, that was most impressive. Your Pokémon believed in you and did their best to earn you the win. Even though I've lost, I find myself with this silly grin!"
"You're quite something, youngster. I like how you and your Pokémon earned the win by working as one. That's what makes you so strong. Ahahaha! I think that you can go as far as you want."
"The Trainer in the next room is a much more formidable opponent. You really must have your wits about you. However, I think you can go as far as you desire. Ahahaha!"
In the anime
Bertha, along with Cynthia, the champion of the Elite Four, was mentioned in An Elite Meet and Greet! by Lucian and by Officer Jenny in A Trainer and Child Reunion!.
She made her debut in An Elite Coverup!, where she, along with Ash and his friends, helped Roland get his Magby back from Team Rocket. After chasing them in her jeep, the group battled the trio and rescued Magby.
Later, Bertha battled Ash and his Torterra but defeated him relatively easily. Afterward, she advised him to be more observant in battles.