You know what, if it's a crazy situational counter you want, it's a crazy situational counter you'll get.
Mantyke (M) @ Eviolite Trait: Swift Swim
EVs: 156 HP / 200 SAtk / 120 SDef
Calm Nature (+SDef, -Atk)
- Rest
- Rain Dance
- Hidden Power [Flying]
- Toxic
Now why does this work? Easy! It resists or takes barely anything from 7/8 of the moves selected. The only problematic scenario whatsoever is it switching into a Snover's Blizzard with Stealth Rock up. Okay, so Snover does 7 damage with Blizzard (9 with max damage). Mantyke has 23 HP, takes 5 HP from Stealth Rock, and loses 1 HP a turn from Hail. It goes like this...
184 SpAtk Snover Blizzard vs 156 HP/120 SpDef Eviolite Mantyke (+SpDef) : 30.43% - 39.13%
Possible HP Damage: 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 9
Mantyke has full health, switches into a Snover max damage Blizzard with Stealth Rock up, and takes Hail damage
23 - 5 - 9 - 1 = 8
Snover uses Blizzard again (no max damage!) and Mantyke uses Rain Dance!
8 - 7 = 1
Mantyke has 3 HP left, and now outspeeds Snover (11 Speed min goes to 22 with Swift Swim...Snover has 21).
HP Flying OHKOes Snover (75% of the time, and Blizzard is now only 70% accurate...so Mantyke comes out on top here 82.5% of the time...if you really care about that you can use HP Fire instead of HP Flying to guarantee that but I really didn't want to as I want to maintain some shred of pride at the end of this)
There are now three turns of Rain left.
Mantyke uses Rest, Staryu uses Hydro Pump (rain boosted)
Two turns left
Mantyke is asleep, Staryu uses Hydro Pump (rain boosted)
One turn left
Mantyke is asleep, Staryu uses Hydro Pump (rain boosted)
Zero turns left
Staryu outspeeds, uses Psychic, Mantyke uses Rest
and GG from this point I guess
196 SpAtk Staryu Hydro Pump vs 156 HP/120 SpDef Eviolite Mantyke (+SpDef) : 17.39% - 26.09%
Possible HP Damage: 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6
^ Rain
196 SpAtk Staryu Hydro Pump vs 156 HP/120 SpDef Eviolite Mantyke (+SpDef) : 13.04% - 17.39% (6-8 hits to KO)
Possible HP Damage: 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4
^ No Rain
196 SpAtk Staryu Psychic vs 156 HP/120 SpDef Eviolite Mantyke (+SpDef) : 17.39% - 21.74%
Possible HP Damage: 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5
The only way Staryu can KO Mantyke in this scenario is if it gets three max damage roll Hydro Pumps in a row when it's raining and a max damage psychic (6+6+6+5=23) but the odds of that are...7.8125*10^-4%!!