Oooh I also had a really good idea of what to give Tetra for this! Malamar!
It's said that Malamar's hypnotic powers played a role in certain history-changing events. When it comes to strong hypnosis, there’s an endless number of people who utilize Malamar for their nefarious deeds. The edges of its fins are supremely sharp. It wields the most compelling hypnotic powers of any Pokémon, and it forces others to do whatever it wants.
I thought this would be fitting for her cunning personality, and it has some kick to it if you give it the Contrary ability, making it a formidable foe.