Ever since the release of Primarina, Azumarill has struggled to find a place in the metagame, hence its C Rank in OU while Primarina sits comfortably in the A Rank. Primarina has way more tools in its arsenal: stronger special defense, more reliable setup in Calm Mind, Draining Kiss to keep Primarina healthy throughout a game and chip away at its switch-ins, Psychic Noise to prevent the opponent from healing and dismantle fat (which pairs nicely with Liquid Voice), stronger STAB moves with Moonblast and Surf, 60 Speed allowing it to speed creep Kingambit on faster sets, slow pivoting with Flip Turn, and so on. So that raises the question: what niche can Azumarill fill? Well, there's 3, but only 1 is consistent.
Whirlpool + Perish Song: Our team briefly discussed it, as it would be able to trap the likes of Dondozo and other bulky waters. In turn, it would open up something like SD Gliscor. However, the meta is far too strong for Azumarill to stay alive for Perish Song's duration, and there are so many viable Pokemon with pivot moves that would blank this Azumarill set.
Belly Drum: On the right team it could work, but Gliscor does not make for a good partner. Belly Drum sets would work better on Screens HO, as Azumarill needs the extra bulk to safely get the Belly Drum off. In general, this set still struggles because of Ogerpon-Wellspring blocking its Aqua Jet and threatening to OHKO.
Choice Band: The best niche that Azumarill can carve itself with the presence of Primarina. While Aqua Jet itself is quite weak, it does enough damage with Choice Band Azumarill's 654 Attack. Ogerpon-Wellspring is no longer a safe switch-in, as Play Rough is a guaranteed OHKO. Both its STAB moves are very spammable, and Azumarill now has access to Knock Off, which pairs really well with hazard stacking from Gliscor while also being a great midground move.
Azumarill enjoys the extra bulk over speed, as it can live many strong attacks in a pinch and fire off a strong attack in response (e.g. +1 Acrobatics from Attack Booster Roaring Moon). 16 Speed EVs can help against uninvested Kingambit, Ursaluna, and other base 50 speed Pokemon. Tera Water is great offensively and defensively, as it boosts your damage of Aqua Jet and Liquidation while retaining a strong Water typing defensively.
Gliscor @ Toxic Orb
Ability: Poison Heal
Tera Type: Water
EVs: 244 HP / 248 Def / 16 SpD
Impish Nature
IVs: 29 Spe
- High Horsepower
- U-turn
- Spikes
- Protect
Gliscor is the physically defensive pivot of the team. Spikes and Protect were going to be on the set no matter what, as you requested Spikes to be paired with Gliscor and Protect is necessary to keep Gliscor healthy throughout the game while being a great Choice-locked scouter. I tested many variations of the other two moves, and I settled on High Horsepower and U-turn. High Horsepower is nearly as powerful as Earthquake while not being reduced in damage by Grassy Terrain from Rillaboom. It hits hard and is fairly reliable; I originally had Knock Off but it made Gliscor too passive and easy to set up on. U-turn helps bring in Azumarill, Darkrai, and Dragapult more safely. 244 HP EVs maximize the amount of health Gliscor recovers from Poison Heal, and 16 Special Defense EVs allow Gliscor to survive both a Choice Specs Enamorus's Moonblast and Life Orb Deoxys-S's Psycho Boost from full health. 29 Speed IVs is my special flavor on U-turn Gliscor sets; it allows you to get a slow U-turn on opposing Gliscor while still outspeeding Jolly Kingambit and Pokemon trying to speed creep. Tera Water is nice for weather matchups to make sure you aren't dependent solely on Azumarill and Rillaboom. If you aren't requiring Spikes on Gliscor, try Toxic instead as it eases some matchups.
Rillaboom @ Choice Band
Ability: Grassy Surge
Tera Type: Grass
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Grassy Glide
- Wood Hammer
- Knock Off
- U-turn
When I initially saw your suggested core, Rillaboom popped into my mind, and I wasn't too sure why at first. After all, it heals up the damage your opponent takes from hazards, which seems like a poor synergy. I tested several versions of the team with bulky Assault Vest Rillaboom, but it felt inconsistent and the team felt too slow. I tried without Rillaboom, and it felt even worse. Eventually, I realized the reasons why Rillaboom works so well with Azumarill and Gliscor. As I mentioned before, one of Azumarill's biggest threats is Ogerpon-Wellspring, and the ogre also threatens Gliscor. Rillaboom is one of the strongest checks to Ogerpon-Wellspring, as it can come in and eat a hit while threatening back with Grassy Glide for huge damage and, more commonly, U-turn to potentially OHKO while punishing an opponent's switch and generating momentum. With Choice Band Rillaboom specifically, it forms a nasty priority core with CB Azumarill Aqua Jet + CB Rillaboom Grassy Glide. This combination is valuable late-game, as in an ideal game, your hazards are up and at least one of Azumarill and Rillaboom are alive. The chip accumulated throughout a game allows for Aqua Jet / Grassy Glide cleaning up the opponent's team. Additionally, U-turn brings in the team's threats more easily, Knock Off hits hard and removes items to help with the hazard stack, and Wood Hammer is a phenomenal breaking tool. Tera Grass maximizes both priority damage and breaking damage depending on the matchup.
Heatran @ Air Balloon
Ability: Flame Body
Tera Type: Ghost
EVs: 248 HP / 208 SpD / 52 Spe
Calm Nature
IVs: 0 Atk
- Magma Storm
- Earth Power
- Taunt
- Stealth Rock
Heatran rounds out the team nicely. It is a specially defensive Stealth Rock user that rounds out the Fire-Water-Grass core and helps against some of the team's biggest threats in Kyurem, Enamorus, Dragapult, Gholdengo, and so on. Air Balloon is really nice as a one-time guaranteed switch-in Kyurem and Enamorus, both of which you threaten out and set up Stealth Rock on to punish them later. Tera Ghost is nice as an emergency spinblock and Fighting immunity. Taunt helps take out Galarian Slowking if you land a Magma Storm on it switching in, and in general Taunt is nice to stop setup. 52 Speed EVs put Heatran at 203 Speed, which will outspeed Adamant Kingambit / Ursaluna / etc. while increasing your odds of outspeeding Skarmory (much like Zamazenta, Skarmory have been hitting speeds more than the suggested 200 Speed to outspeed and win the 1v1 against opposing Skarmory, so you want to try to get the jump on them. 203 Speed was the most I was comfortable going, as its Special Defense is still able to hit the jump point of 330). As for its ability, I think both are great, but Flame Body probably has more use as it can punish contact physical attackers like Roaring Moon in a pinch. Flash Fire is really only important for the most common Iron Moth set (Fiery Dance, Sludge Wave, Tera Blast Ground, Substitute) as you are immune to all attacks with an Air Balloon intact, but you can revenge kill Tera Ground Moth with Rillaboom or Azumarill. Other Fire-type Pokemon that can be walled like Ceruledge aren't common enough to warrant Flash Fire, and the damage boost is not a big deal. Feel free to use either ability, I think both are fine.
Darkrai @ Heavy-Duty Boots
Ability: Bad Dreams
Tera Type: Poison
EVs: 80 HP / 176 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Dark Pulse
- Sludge Bomb
- Focus Blast
- Nasty Plot
Pretty standard Darkrai here. Heavy-Duty Boots is pretty necessary for opposing hazard stack teams as well as Glimmora's Toxic Spikes which would otherwise shred this team apart. Nasty Plot allows Darkrai to be a set-up win condition for the team. Sludge Bomb and Focus Blast are the preferred coverage moves for the team, as Sludge Bomb hits Grass- and Fairy-types while Focus Blast hits Dark- and Steel-types. Tera Poison gives Darkrai a resistance to Fairy and Fighting, boosts the power of Sludge Bomb, and allows the team to absorb Toxic Spikes if needed. The EV spread allows Darkrai to hit hard while making better use of its solid defenses.
Dragapult @ Heavy-Duty Boots
Ability: Infiltrator
Tera Type: Fairy
EVs: 76 Atk / 180 SpA / 252 Spe
Naive Nature
- Dragon Darts
- Hex
- U-turn
- Will-O-Wisp
There's not too much to explain here; Boots Pivot Dragapult is amazing. Speed control and fast pivot for the team, amazing STAB types, ability to cripple physical attackers with Will-O-Wisp, Infiltrator to attack through Substitute and Screens, Heavy-Duty Boots to allow longevity and ease of being brought in during the game, and mixed offenses to make it more difficult to switch into. Tera Fairy helps greatly against the Dragon and Dark attacks that dominate the tier.
I hope you enjoy the team!
Teambuilding Lab Application 
Rules:
