Azumarill is a pretty solid counter to Walking Wake as noted by the first
solo guide videos I could find using it, but I wanted to see if it could beat Iron Leaves and other bosses that I didn't try Azumarill on initially. Iron Leaves is very frustrating to fight with its random Swords Dances, and being a 5 star raid makes its move choices seem rather random still.
Since it has a stat reset at 50% HP while the shield goes up at 55% HP, I decided to try to set up Mud-Slaps to hopefully dodge all the +6 attacks and go for a big tera hit before the shield to beat it and skip the reset like Stored Power does, or alternatively break the shield while above 25% HP and then kill on the free turn afterwards while I still have my boosts. I ended up choosing to go for Tail Whips early on as well to get the damage I needed out of Play Rough, but a super effective tera* would have worked as well. I also put Bright Powder on Azumarill over Shell Bell as I needed to dodge most attacks and it would be over in one hit ideally. Initially I was trying Sap Sipper, but for this clear I needed Huge Power for upfront power. Deaths would be fine since I would only need about 2/3 free turns to set up Belly Drum, an attack cheer, and hit Play Rough depending on if I got speed lowering CPUs or not.
+6 252+ Atk Huge Power Tera Fairy Azumarill Play Rough vs. -2 Lvl 75 0 HP / 0 Def Tera Psychic Iron Leaves: 2804-3300 (57.6 - 67.9% of 20x hp boss) -- guaranteed 2HKO
+6 252+ Atk Huge Power Tera Fairy Azumarill Atk Cheer Play Rough vs. -2 Lvl 75 0 HP / 0 Def Tera Psychic Iron Leaves: 4204-4948 (86.5 - 101.8% of 20x hp boss) -- 12.5% chance to OHKO
On my first attempt with this strategy, I ended up getting Quark Drive to boost SpD which must have been Calm or an imperfect Atk IV or something. I also had Intimidate Tauros letting me survive some early turns including a random Megahorn choice. I then set up some Tail Whips, but then Heracross started landing some Bulk Up boosted Megahorns, and so the NPCs ended up destroying half of Iron Leaves's health bar themselves.
EDIT: I forgot Intimidate guarantees Quark Drive boosts SpD.
I barely managed to survive another hit** and it must have not gone to +6 yet, so I got one Mud-Slap in before dying. Then I set 2 more Mud-Slaps up smoothly for 45% accuracy dodges, and got my Belly Drum in before Play Roughing to break the shield. The AI actually ended up getting the killing blow immediately afterwards, so I did not get the text saying Iron Leaves fainted on my screen which will be my next goal. But there's another event boss win for Azumarill.
*Alternative options: It turns out a SE 100BP Stellar Tera Blast with 1.2x STAB does the exact same damage as an 80 BP 1.5x STAB Tera Blast, because the BP difference of 1.25x exactly makes up for it.
+6 252+ Atk Huge Power Tera Dark Azumarill Atk Cheer Tera Blast (80 BP) vs. Lvl 75 0 HP / 0 Def Tera Psychic Iron Leaves: 2804-3300 (57.6 - 67.9% of 20x hp boss) -- guaranteed 2HKO
+6 252+ Atk Huge Power Tera Stellar Azumarill Atk Cheer Tera Blast (100 BP) vs. Lvl 75 0 HP / 0 Def Tera Psychic Iron Leaves: 2804-3300 (57.6 - 67.9% of 20x hp boss) -- guaranteed 2HKO
**The Leaf Blade I took did 198 damage on the last turn of the defense cheer. Iron Leaves was probably only at +2 (2 SDs after 2 Tauros Intimidates) if it was Calm with imperfect Attack as far as I can tell.
+2 Lvl 75 0- Atk Tera Psychic Iron Leaves Leaf Blade vs. 252 HP / 0 Def Azumarill through Def Cheer: 192-228 (47.5 - 56.4%) -- 83.2% chance to 2HKO
EDIT: I got the proper kill screen with a new team of Weavile/Drifblim/Gardevoir. Here I went to -2 Defense with a Weavile Leer and used Pixie Plate to guarantee breaking the shield from 55% without an attack cheer, so I only needed to Belly Drum once between setup and not risk Weavile chipping to the point of a reset, though I think an early death caused the shield to go up slightly early anyway. Drifblim's burn also helped survival while Gardevoir's Life Dew was more relevant near the start.
+6 252+ Atk Fairy Feather Huge Power Tera Fairy Azumarill Play Rough vs. -2 Lvl 75 0 HP / 0 Def Tera Psychic Iron Leaves: 3364-3958 (69.2 - 81.4% of 20x hp boss) -- guaranteed 2HKO
The shield takes 80% damage from matching tera, so Azumarill hits for at least 55% with a minimum roll with the 20% item boost.