lol i was looking for something too and the best i could do was CM LatiasI had to dig deep for a mon that covers most of the threats listed above, and it came down to this:
Slowking @ Assault Vest
Ability: Regenerator
EVs: 248 HP / 176 Def / 84 SpA
Modest Nature
- Scald
- Psychic / Future Sight
- Fire Blast
- Dragon Tail / Ice Beam
AV Slowking is a strong answer to both Nidos and most bulky Water-types in the tier; the only ones that can 2HKO on the switch are Mega Blastoise and the odd Modest Primarina with Stealth Rock up 29% of the time. The EVs are the ones listed on its analysis: to be able to survive 2 Stone Edges from Mega Aerodactyl (you're 2HKOed after Rocks but we can double into Klefki/whatever else we have on this team) while the remaining EVs are put into SpA so you can hit decently hard.
Psyshock was an option listed but I feel Psychic or Future Sight would be better since we shouldn't really let this thing be fighting Conkeldurr 1v1 nor are we that worried about most CM users that we'd want to run Psyshock. Future Sight is kinda cool since it's capable of forcing switches, which is what any team that runs Spikes appreciates. Fire Blast hits Celebi, Bisharp, and Scizor, and though Dragon Tail isn't slashed on the analysis as it is here, I feel it's worthy of considering Ice Beam over it just so we have something else that can lure in Hydreigon while fending off Latias. It takes some of the pressure off Klefki...I mean Dragon Tail still hurts both of them, Ice Beam just does more lol.
i never even thought of slowking but now that you mention it it's a lot better than Latias in terms of checking the above threats
what we need now tho is some serious speed as the fastest mon on our team is Hax which is pretty depressing tbh