Blastoise (M) @ Leftovers
Trait: Torrent
EVs: 252 HP / 168 SAtk / 88 SDef
Calm Nature (+SDef, -Atk)
- Foresight / Mirror Coat
- Hydro Pump
- Rapid Spin
- Refresh
ADV Blastoise is a generally a much worse Swampert with access to Rapid Spin and some more Speed. Hydro Pump coming from 248 Special Attack will kill even max HP Gengar in two hits and is very, very likely to finish off Skarmory in three, even if it uses Protect every second turn, whereas Gengar needs 328 SpAtk to even have a slight chance to do the same to Blastoise with Thunderbolt.
168 SpA Blastoise Hydro Pump vs. 252 HP / 0 SpD Gengar: 173-204 (53.3 - 62.9%) -- guaranteed 2HKO after Leftovers recovery
168 SpA Blastoise Hydro Pump vs. 252 HP / 252 SpD Skarmory: 135-159 (40.4 - 47.6%) -- guaranteed 3HKO after Leftovers recovery
Foresight allows Rapid Spin to hit through Ghosts and mighty be useful since Hydro Pump only has a shoddy 64 % chance of hitting twice in a row. Mirror Coat punishes Gengar for using Thunderbolt and can serve as a last resort option against non-Roar/Toxic/HP Suicune. Refresh prevents certain death from status conditions such as Skarmory's Toxic or Gengar's Will-o-Wisp.
Other options include Ice Beam/Icy Wind, Counter, Roar, Haze, Toxic, Rain Dance, Protect and Yawn, with both Haze and Yawn having compatibility issues with Foresight and Refresh. I guess you could drop either of Foresight and Mirror Coat anytime and Refresh too, if you have a Magneton as well. In this case Protect/Rapid Spin/Surf/Toxic with more Defense might be the best bet:
Blastoise (M) @ Leftovers
Trait: Torrent
EVs: 252 HP / 220 Def / 36 SDef
Bold Nature (+Def, -Atk)
- Protect
- Rapid Spin
- Surf
- Toxic
362 HP / 320 Def enables it to survive 3 Choice Band Metagross Meteor Mashes or 2 Choice Band Tyranitar Rock Slides.
252+ Atk Choice Band Metagross Meteor Mash vs. 252 HP / 220+ Def Blastoise: 102-120 (28.1 - 33.1%) -- 91.1% chance to 4HKO after Leftovers recovery
252+ Atk Choice Band Tyranitar Rock Slide vs. 252 HP / 220+ Def Blastoise: 153-180 (42.2 - 49.7%) -- guaranteed 3HKO after Leftovers recovery
The only other idea that possibly comes to mind is a
very poor imitation of VIL's Charizard with less Spd and SpAtk and a far worse typing:
Blastoise (M) @ Petaya Berry / Salac Berry
Trait: Torrent
EVs: 4 HP / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk) / Timid Nature (+Spd, -Atk)
- Hydro Cannon / Toxic / Ice Beam
- Hydro Pump
- Rain Dance / Toxic / Ice Beam
- Substitute
+1 252+ SpA Torrent Blastoise Hydro Cannon vs. 4 HP / 0 SpD Blissey in Rain: 523-616 (80.2 - 94.4%) -- guaranteed 2HKO after Leftovers recovery