NFE Z-Hypnosis Belly Drum Sweeper (Poliwhirl) @ Psychium Z  
Ability: Water Absorb  
EVs: 252 Atk / 4 SpD / 252 Spe  
Jolly Nature  
- Hypnosis  
- Belly Drum  
- Ice Punch / Waterfall
- Earthquake / Waterfall
Optional Move(s): 
- N/A
Optional Item(s): 
- Eviolite
- Normalium Z
Optional Ability: 
- Swift Swim
Optional EV/IV(s):  
- N/A
Overview: With access to Hypnosis and Belly Drum, Poliwhirl is a niche cleaner capable of cleaning through weakened teams after setting up. However, it requires two turns to set up. It also only has a 40% chance to successfully fire off a Z-Hypnosis and then Belly Drum, which makes it unreliable as a set up Pokemon. In addition, it also faces heavy competition as a physical Water-type attacker from Marill and Croconaw. Overall, due to its average Speed tier, two turn setup, and being relatively easy to KO after Belly Drum, it's a high maintenance sweeper that takes a well seasoned trainer to successfully pilot. 
Z-Hypnosis + Belly Drum Sweeper: By finding a passive pokemon to prey on, such as Mareanie or Mudbray, Poliwhirl can fire off Z-Hypnosis to begin its setup process to get off its Belly Drum. Z-Hypnosis boosts Poliwhirl's Speed by +1, while also putting the opposing Pokemon to sleep, which gives it a setup opportunity for Belly Drum. With its perfect neutral coverage in Ice Punch and Earthquake, switching into it is near impossible at max attack, and with the +1 speed boost from Z-Hypnosis, it can outspeed the entire unboosted metagame. Ice Punch provides good coverage against Flying- and Grass-types such as Servine, Roselia, Staravia, Togetic, and Vullaby. Earthquake is primarily used to OHKO Mareanie at +6 Attack, but also hits Electric-, Fire-, Poison-, and Steel-types such as Electabuzz, Monferno, Alolan Grimer, Combusken, Klang, and Metang. Using a STAB attack in Waterfall may seem obvious, but because of the coverage that Earthquake and Ice Punch provide, the move is rather niche as it forces you to pick between beating most Grass-types or Mareanie.
Checks and Counters: Before setup, Electric- and Grass-types such as Electabuzz and Servine can threaten Poliwhirl with their STAB attacks, effectively forcing it out or KO'ing it. Poliwhirl's mediocre bulk also leaves it prone to being revenge killed after setup by faster Choice Scarf users, such as Haunter or Frogadier. On the same note, a weakened Poliwhirl is prone to being beaten by priority, such as Monferno's Mach Punch and Vacuum Wave, or Pawniard's Sucker Punch. Bulkier physical walls, such as Quilladin, Tangela, and Dusclops can take a hit even at +6 and threaten it back.