Assuming a set of:
Inteleon @ Scope Lens
Ability: Sniper
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Focus Energy
- Hydro Pump
- Ice Beam
- Mud Shot
252 SpA Sniper Inteleon Mud Shot vs. 252 HP / 252 SpD Toxapex on a critical hit: 168-198 (55.2 - 65.1%) -- guaranteed 2HKO after Black Sludge recovery
252 SpA Sniper Inteleon Mud Shot vs. 252 HP / 136+ SpD Toxapex on a critical hit: 162-192 (53.2 - 63.1%) -- guaranteed 2HKO after Black Sludge recovery
If Toxapex doesn't go full Special Defense, Mud Shot from Inteleon can 2HKO. A Full SpDef Toxapex takes this:
252 SpA Sniper Inteleon Mud Shot vs. 252 HP / 252+ SpD Toxapex on a critical hit: 153-180 (50.3 - 59.2%) -- 74.6% chance to 2HKO after Black Sludge recovery
So not exactly the most reliable counter, but the rest of your post still stands.