Reserving CB Azu + NP Thundy
Azumarill @ Choice Band
Ability: Huge Power
EVs: 160 HP / 252 Atk / 96 Spe
Adamant Nature
- Waterfall
- Play Rough
- Aqua Jet
- Superpower
or Knock Off
Thundurus (M) @ Leftovers
Ability: Prankster
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Nasty Plot
- Thunderbolt
- Hidden Power [Ice]
or [Flying] (Run 4 HP EVs with that BC IV drop)
- Thunder Wave
or Focus Blast
More or less a textbook example of two 'mons that beat each others checks and counters. CB Azu can come in on and break most grounds (In terms of support, most notably Hippo and Excadrill). Furthermore, it can revenge kill Latis, lure in and cripple Ferrothorn, smack T-Tar and even KO slow Specs Mag if push comes to shove. It also hard checks Weavile as a cherry on top.
In return, Thundy can immediately threaten bulky waters that wall Azu - which doesn't mean Quag or Swampert as Azu can 2HKO both. Additionally, it can break Ferrothorn if carrying Focus Blast, and past bulky grasses if running HP Flying, or possibly being able to paralyze set-up sweepers so Azumarill can RK them.
However, this core is heavily threatened by fast electrics, along with having to balance utility and coverage. Also, it's not notably air tight on the defensive spectrum, so teammates will be appreciated.
Azumarill's EVs allow it to speed creep things speed creeping defensive Tyranitar - yes, the latter is not reliable and will rarely come into use, but IMO there's not real reason to not run more speed as more HP will just result in a non-SR number. If you don't want such a speed creep and want a spread that does a bit more, then run 160 HP / 252 Atk / 4 SpD / 92 Spe for Download Porygon2s, but IMO that's even less relevant than things speed creeping defensive T-Tar.