It's good to know that Hidden Power doesn't exist in Gen 9, thank you :3

Heatran would probably stand a slightly better chance against some of them with Tera types, I reckon. Tera Grass would give it a Ground resistance (Flying would make it immune, but then it'd be vulnerable to Rock-type moves and have no way to deal significant damage), and Grass-type Tera Blast would safely 2HKO Hippowdon, for example. Even a mono-Fire typing would allow it to survive Hippowdon's Earthquake and, with some luck, 2HKO with Fire Blast.