You didn't mention investments but to have an idea...First turn a Staraptor managed to OHKO a defense and HP invested bulky Drapion on full health with Double Edge. Then second turn it OHKO'd a defense and HP invested bulky Gliscor on full health with another Double Edge. Then he proceeded to OHKO my last Pokémon (max HP Azumarill on full health) with a Porygon Z's Hyper Beam. I didn't get a move in, basically.
I'm not sure how any of those three damage rolls and OHKO scenarios are physically possible. Choice Band Adamant Staraptor?? I dunno. But hey, that's Pokémon. I've learned something today.
252+ Atk Choice Band Reckless Staraptor Brave Bird vs. 252 HP / 252+ Def Drapion: 246-291 (71.5 - 84.5%) -- guaranteed 2HKO
252+ Atk Choice Band Reckless Staraptor Brave Bird vs. 252 HP / 252+ Def Gliscor: 226-267 (63.8 - 75.4%) -- guaranteed 2HKO after Poison Heal
252 SpA Adaptability Porygon-Z Hyper Beam vs. 252 HP / 0 SpD Azumarill: 212-250 (102.4 - 120.7%) -- guaranteed OHKO
I'm surprised Porygon-Z can OHKO Azumarill with Hyper Beam with no items.