Moves
Belly Drum is used for boosting Azumarill's Attack stat at the cost of half of its health. Aqua Jet is a priority move and Azumarill's main means of damaging faster opponents. Play Rough is Azumarill's secondary STAB attack and achieves decent neutral coverage with Aqua Jet. The choice between the last three moves depends on your team structure; Waterfall is an alternative STAB option that is more accurate than Play Rough, Knock Off removes troublesome items such as Eviolite and Rocky Helmet, and Superpower gives Azumarill the coverage necessary to get past Ferrothorn.
Set Details
This EV spread maximizes Azumarill's Attack and overall bulk whilst also giving an even HP stat at level 50; this is important, as it puts it into Sitrus Berry's activation range after Belly Drum is used. The leftover 4 EVs are put into Speed to allow Azumarill to outpace opposing base 50s, such as Chansey, Aggron, Tangrowth and Mega Mawile. The EV spread can be tweaked depending on what you want Azumarill to outspeed; 228 HP / 252 Atk / 12 Def / 12 SpD / 4 Spe with an Adamant nature is the safest due to the large amount of bulk, but points can be moved from HP into Speed to outpace faster threats; for instance, 132 HP / 252 Atk / 124 Spe with an Adamant nature allows Azumarill to outspeed Pokemon with an uninvested base 65 Speed stat while still keeping Azumarill's HP at an even number. This jump point encompasses Pokemon such as Scizor, Tyranitar, Abomasnow, and Clefable, greatly improving Azumarill's matchup against them and making it less dependent on Aqua Jet. Huge Power is Azumarill's preferred ability for all of its sets, as it doubles Azumarill's Attack stat, bringing it from a middling 112 to a much more impressive stat of 224, allowing it to do much greater damage than it would be capable of with either of its other two abilities. Sitrus Berry synergizes well with Belly Drum, recovering 25% of Azumarill's health when it has 50% HP or less remaining. This leaves Azumarill at the maximum possible amount of HP after boosting and makes it more likely to survive oncoming attacks during its sweep.
Usage Tips
Despite its bulk, Belly Drum Azumarill can sometimes be a risky Pokemon to play with due to the health lost while setting up and the free turn required to do so. However, there are several Pokemon in the format that cannot significantly hurt Azumarill with certain attacks and as such make good setup opportunities: examples include Heatran, Mega Sableye, Hydreigon, Tyranitar, and Garchomp locked into Outrage or Rock Tomb. Because the list of setup opportunities for Azumarill is quite specific, some scouting is usually required before Azumarill can commit to a sweep by clicking Belly Drum. However, if you have other viable win conditions, Azumarill can be used effectively as an early hole puncher. At +6, Azumarill's Aqua Jets are strong enough to do solid damage to a significant portion of the format. Due to this, it makes a fine win condition, especially against slower teams. However, Azumarill still has difficulty breaking through Pokemon such as Cresselia, Mega Venusaur, Zapdos, Ferrothorn, and Porygon2 without prior damage. This is where scouting can be useful: if these threatening Pokemon can be knocked out or sufficiently damaged, Azumarill can easily clean up with its moves. While clicking Belly Drum and spamming Aqua Jet can be tempting, there are occasions where Azumarill can hold off on boosting and save its item (and its health) for a later opportunity. Aqua Jet makes Azumarill a fairly effective revenge killer, and there will also be occasions where it's wiser to simply damage the opponent rather than go for the boost and risk having Azumarill faint.
Team Options
While Azumarill is strong, its base 50 Speed is lackluster and makes it easy to revenge kill. As such, plenty of faster Pokemon can stop its sweep, either by resisting Aqua Jet or possessing faster priority. Azumarill's main concern is opposing Grass-types: if they outspeed it, they are capable of easily taking a +6 Aqua Jet and OHKOing in return with a STAB Grass-type move. Azumarill also has trouble with Talonflame, as Choice Band Brave Bird easily does enough damage to OHKO Azumarill after the HP loss from Belly Drum. For these reasons, Heatran makes a very effective teammate for Azumarill, as it resists both of Talonflame's STAB types and also walls many common Grass-types, including Venusaur, Ferrothorn, and Roserade. They also synergize defensively and Azumarill can beat Heatran's answers, including Garchomp, Landorus-T, and Conkeldurr, in return. Electric-types make good teammates for Azumarill thanks to their above-average Speed stats, passable physical bulk, and resistance to priority moves such as Bullet Punch and Brave Bird. Mega Manectric in particular excels at this role thanks to its ability, Intimidate, which allows it to switch in on priority for Azumarill and remove the threat with its coverage. Pokemon like Garchomp, Mamoswine, Excadrill, and Landorus-T are useful supporters for Azumarill thanks to their positive matchup against Electric-types with their immunity to Electric-type moves and STAB Earthquake; however, Garchomp and Landorus must take care around carriers of Hidden Power Ice. Azumarill doesn't like to take damage if it can avoid it, as too much can keep it from successfully getting a Belly Drum off. For this reason it appreciates Volt Switch or U-turn support from teammates such as Assault Vest Landorus-T and Raikou. Finally, this variant of Azumarill benefits a lot from Mega Gengar, which can switch in on Grass- and Poison-type moves and trap and remove problematic Pokemon such as Zapdos, Cresselia, Mega Slowbro, Mega Kangaskhan, Mega Venusaur, Ferrothorn, and Serperior for Azumarill. In turn, Azumarill can dispatch Dark-type Pokemon that threaten Gengar, such as Hydreigon and Tyranitar.