From one Belly Drum Sweeper to another. While Azumarill is far from a one trick pony as opposed to Eiscue, Belly Drum boosted blows from it are terrifying. 100/80/80 defenses attached to the fantastic Water/Fairy typing makes Azumarill fairly bulky, and Huge Power turns a pathetic base 50 attack into a force of destruction.
Rabbit of Caerbannog
Azumarill
Ability: Huge Power
Item: Sitrus Berry
-Belly Drum
-Aqua Jet
-Play Rough
-Liquidation/Knock Off
Nature: Adamant (+ATK, -SATK)
EVs: 4 HP/252 ATK/252 SPE
Belly Drum Azumarill is a threat that is meant to come in late game where all of its checks are weakened or removed so you can go for the setup and sweep. Aqua Jet is the bread and butter of the set, giving Azumarill STAB priority and forcing your opponent to run priority of their own if they want to outspeed Azumarill in the event they have no resists.
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 4 Def Corviknight: 304-358 (76 - 89.5%) -- guaranteed 2HKO after Leftovers recovery
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 112 HP / 20 Def Spectrier: 465-547 (126 - 148.2%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 96 HP / 0 Def Regieleki: 550-648 (169.2 - 199.3%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 4 Def Tornadus-Therian: 381-448 (105.2 - 123.7%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 0 Def Weavile: 451-532 (160.4 - 189.3%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 4 Def Celesteela: 309-364 (77.6 - 91.4%) -- guaranteed 2HKO after Leftovers recovery
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 120 HP / 0 Def Scizor: 318-375 (102.2 - 120.5%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 0 Def Dragapult: 201-237 (63.4 - 74.7%) -- guaranteed 2HKO
The Numbers are clear. If the target is frail, it is falling to Aqua Jet. If the target is frail with a resist or Bulky, it will be 2HKOd. While Belly Drum does cut into Azumarill's bulk, Sitrus Berry does counteract it somewhat, and even at 75%, Azumarill can often stomach a hit. Play Rough is STAB for the moments where Aqua Jet is just NOT an option. Final option falls between Liquidation for strong STAB, or Knock Off, which hits everything that resists Azumarill's STAB combo.
Other Options
Perish Song + Whirlpool -gimmick set, but Azumarill's bulk is decent and can nab a KO against anything that can't KO it in time.
Checks and Counters
The number of pokémon that can stomach Azumarill's moves after a Belly Drum is a short list: Unaware Quagsire, Unaware Clefable, G-Weezing, and Pyukumuku (Ferrothorn and Amoongus are OHKOd by +6 Knock Off without max investment and Toxapex has an 81% chance for the OHKO from Knock Off). Pokémon that null/resist water attacks can try to KO Azumarill before it switches to Play Rough or Knock Off in the face of a predicted Aqua Jet, but Azumarill is fairly bulky. Priority moves are in a similar boat. Rillaboom is fantastic, having STAB Super Effective priority and can easily stomach an Aqua Jet. Kartana and Tapu Bulu are in a similar boat, just without the priority as is Tangrowth to a lesser extent.
Burning Azumarill is a great way to stop it dead in its tracks, as that really eats into its famed offense. The standard Water checks will work against Azumarill after a burn as will most physically bulky walls.