A boss that can't be safely debuffed may be one of the hardest challenges Azumarill has faced yet, and dealing with a stat reset phase without any safety net is intense. A small mercy is that Empoleon likes to spam Ice Beam against Azumarill for whatever reason. It still uses a fair share of Flash Cannon and Grass Knot though, but I never saw Surf.
As predicted, the choice of AI allies is crucial to success for a non-Unaware mon, with Intimidate allies very easily giving Empoleon +2 and +4 after the initial Blizzard. My initial attempts were with Arboliva/Clodsire/Weavile but after Weavile kept ruining early setup with Leer, I decided to try again. I would settle for two goals, getting Gardevoir for consistent Life Dew healing and also getting a team that would screw me over the least with any debuff moves, which I found the AI ended up being forced to use quite a bit after running out of PP. In the end, despite me calling Sylveon one of the bad AI options in an earlier post, I found it to be the most tolerable of the potential debuffers, being more inconsistent at giving Empoleon a power spike thanks to Moonblast's 30% effect rate and only boosting it a net 1 stage at a time.
I ended up tailoring my moveset around this specific AI team as well, running the standard 252 HP/Atk with Shell Bell and Amnesia/Rollout/Defense Curl/Belly Drum. With Arboliva I was trying Rindo Berry to avoid boosted Grass Knots and Aqua Ring for sustain over Belly Drum, but there just wasn't enough time to manage it. Gardevoir can take care of healing duties which makes getting Belly Drum off a lot more reliable. Overall the goal is to maximize Rollout's power for a massive finishing blow as usual, but this time it takes a bit more effort to get to the point where it's possible.
The typical start to the battle is setting 3 Amnesias and Defense Curl to bulk up/boost Rollout/stall out the snow. I go for an early Rollout without Belly Drum, mainly just to chip Empoleon down to 80% HP for the first stat reset phase, and also have that damage go into the shield as it goes up around 5 turns in. The time immediately after the reset is very harsh to survive in though, especially if Empoleon has accumulated any buffs. In this attempt I ended up getting a miss right after the reset to cancel Rollout, so I was able to set up a defense cheer and offense cheer to be preserved after fainting (still works in 3.0.1), and my sacrifice also knocks the timer down to 50% for the second stat reset, meaning any buffs will stick from now on and I don't have to wait around to set up again.
By this point it had gotten to +2 from 2 Moonblast procs, but I was still able to get all the required setup back up thanks to the preserved defense cheer, Empoleon choosing Ice Beam, and Gardevoir's healing. He did get 2 SpD drops from Flash Cannon so I used an extra Amnesia and a heal cheer to make enough room for Belly Drum.
At this point, the last thing to do was press Rollout and watch as the battle played out to victory or death. After Rollout 2, Empoleon hit the 65% HP mark for Iron Defense. Rollout 4 broke the shield (and Empoleon also reset its debuffs pointlessly), and Rollout 5 managed to finish the rest of its health in one shot from the green at full power. Technically I could have used a Water/Stellar Tera Azumarill to break the shield faster/do more damage in this scenario, but this was my Tera Rock one so I did not tera here.