In hindsight I feel like this Pikachu is more or less about even with its previous incarnation, mainly on the bulk side because the 120% damage vs only 70% damage going through the shield makes up the 3000 HP difference and more with a 1.7x damage difference. Maybe the initial Thunders would be harder to survive for Azumarill and would still do more damage than SE Iron Tail, and this clear shows a bit of how that might look.
Firstly I want to shout out NR7/Nrom7 who has done the mentioned Tera Grass Azurill solo here at 12:30, and I learned that the Grass Tera Blast animation is actually pretty fast. I tried Fairy myself and can confirm Pikachu does go for Iron Tails pretty often, unless it can Surf to finish from the red.
However I got mad at Pikachu resetting its debuffs at 65% HP and I wanted to complete the family with a final unique take, and so I brought back the Tera Fighting Marill from Eevee to Focus Punch OHKO another mascot. This would not be as easy without the super effective multiplier though, and it truly came down to the wire.
AI notes: This got pretty messy after multiple attempts of testing. For a while I was running Umbreon/Clodsire/Sylveon but it wasn't really working out. I actually think Umbreon's Thunder Wave does not proc Lightning Rod behind the shield since I saw it spam it for a while while Pikachu was not poisoned, and poison can also get in the way of Synchronize though I didn't see Pikachu ever actually proc it on itself. Sylveon's Moonblast can help counteract the boost, though if too many Moonblasts go through Pikachu might go for Play Roughs. Arboliva would be really nice for Grassy Terrain microheals and Safeguard for early para protection. Mudsdale goes for Rock Smashes if it manages to use 10 High Horsepowers, which makes the required Defense drops much more reasonable to get. Gardevoir is helpful for Life Dew and potentially Synchronize. Bellibolt can Light Screen and control speed to let me get more moves in under lower accuracy.
I rerolled once and got Umbreon/Bellibolt/Garganacl. Garganacl can also contribute to lowering speed with Rock Tomb, though Pikachu only needs to go to -2 to be outsped by Marill. I still ended up passing and looking for some healing/Mudsdale, as HP and defense drops were my main bottlenecks at this point.
My next and final settled set was Arboliva/Haxorus/Mudsdale, with two big hitters I was looking for and managing to become the successful run on my first try with them. With Arboliva I could lighten the load on the health thresholds, though Pikachu missing it and not putting up Grassy Terrain also became an issue, and Mudsdale was also not guaranteed to use up PP if it died, so in general I still went for 6 Mud-Slaps for success. I think Mudsdale still died often though despite my efforts.
In theory Marill's chance of death T1 from spread Surf + def cheer Thunder is not high but existent, usually requiring a top 4/16 Thunder damage roll. I actually forgot this was spread at first and was calculating a much higher chance of death. Generally I started off wanting to die T1 to be able to stack to +2 Defense cheers as well as an offense cheer, but then I realized Pikachu about 2HKOs anyway on life 2 regardless, though this could be avoided with Light Screen. Max SpD could work but then you can't die T1 to stack the defense cheers in the first place, but I guess you still save a turn which is the real reward. Like it's not ideal to spend 3 lives dying to 5 perfect Thunders in the worst case.
I found it works either way if I die T1 or not, the latter just saves a turn of rain but costs an extra cheer if I want to get to +2 defense cheers, which does run me out of all my cheers, so in a way I prefer the former if I have support to prevent the 2HKOs. I guess technically you could also stack 2 attack cheers but I didn't really take this into account until writing this now. Afterwards I generally planned around setting Mud-Slaps and cheers when I can before dying. Bellibolt's Light Screen can help to survive another turn on the second life by only getting 3HKO'd by Thunder after a defense cheer stack or two. Ironically I ran into an issue where Pikachu would try to finish me off with its imperfect accuracy Play Roughs or even Surfs when left at red HP and I couldn't actually end up dying.
0+ SpA Light Ball Tera Water Pikachu Surf vs. 252 HP / 0 SpD Eviolite Marill in Rain with spread damage: 80-94 (23.2 - 27.3%) -- 63.7% chance to 4HKO
0+ SpA Light Ball Tera Water Pikachu Thunder vs. 252 HP / 0 SpD Eviolite Marill through Def Cheer: 230-272 (66.8 - 79%) -- guaranteed 2HKO
0+ SpA Light Ball Tera Water Pikachu Thunder vs. 252 HP / 0 SpD Eviolite Marill through Light Screen and Def Cheer: 153-181 (44.4 - 52.6%) -- 23.4% chance to 2HKO
0+ SpA Light Ball Tera Water Pikachu Thunder vs. +2 252 HP / 0 SpD Eviolite Marill: 174-206 (50.5 - 59.8%) -- guaranteed 2HKO
0+ SpA Light Ball Tera Water Pikachu Thunder vs. +2 252 HP / 0 SpD Eviolite Marill through Light Screen: 116-137 (33.7 - 39.8%) -- guaranteed 3HKO
(Marill has 344 HP and these last 2 add up to 343 which is neat.)
In the final attempt with Mudsdale, I lived T1 and put up the offense cheer on T2 while dying, then set Mud-Slap on T3 and the final defense cheer on T4. On the last turn of rain and the first of my 3rd life, I hit Mud-Slap while getting Thundered below half. I ended up setting up to max Mud-Slaps, then getting 2 Liquidation defense lowers in 3 moves before taking another death, but knew from all the past attempts I still had plenty of time to set up and make something happen.
I got Thundered early to take me below half HP, but I was getting insane Liquidation luck into 3 and 4 Defense drops in 6 moves. Fortunately Arboliva's Seed Sower kicked in just in time on that last move just before the tremendous energy cutscene/10% time triggered, leaving me with a minute to act.
With one last confirmed defense drop to see 5 and seeing Pikachu's health actually chipped down pretty hard very close to the 65% HP threshold, I went for Belly Drum and saw Mudsdale's first Rock Smash on that turn, hoping he won the coinflip but with no time to check. Looking back I don't think an animation played but it was fainted before that turn anyway so I'm not sure if it would have been visible.
With my last input to finally terastallize, Marill still had to tighten its focus and survive through the next attack. Pikachu couldn't strike again with Thunder at -6 accuracy (21%), and Marill's focused punch landed true. Overall it goes to show once again Mud-Slap and raid AI (and the happy accident of cheer preservation/stacking) are very helpful to let you get away with a lot in raids if the circumstances enable it.
The chance of getting the 6 needed 20% Liquidation defense drops in X attempts is a binomial distribution, not accounting for Mudsdale finally getting going. On a failed earlier attempt I noticed I could barely fit in about 14 Liquidations with 3 deaths, so subtract 2 Liquidations for Belly Drum and Focus Punch once set up. That's about a 1.9% for all 6 within that time and about 7% for 5. In this attempt I made it in 5/7, which is a pretty fortunate 0.467%. I think a combination of getting hit, Grassy Terrain healing, and actually getting a viable run with defense drops might have slowed my pace down a bit as well.
+6 252+ Atk Huge Power Tera Fighting Marill Atk Cheer Focus Punch vs. -6 0 HP / 0 Def Tera Water Pikachu: 10105-11889 (95.7 - 112.6% of 50x hp boss) -- 75% chance to OHKO
+6 252+ Atk Huge Power Tera Fighting Marill Atk Cheer Focus Punch vs. -5 0 HP / 0 Def Tera Water Pikachu: 8881-10449 (84.1 - 99% of 50x hp boss) -- guaranteed 2HKO
Accounting for the 70% same-tera shield damage, this kill actually did come down to all the chip damage making it possible, with the health being at about 70% and could have either been a successful -6 or a max roll -5. So I guess even Haxorus doing damage was helpful enough.
Also Tera Blast Grass would have 240 effective power compared to Focus Punch's 225, and is also boostable by Grassy Terrain. So technically Marill does have an actual OHKO route even with the shield in play from 100% HP and without DLC. I just did not want to build a new one, but maybe I would have if I went through all that and missed the actual OHKO.
+6 252+ Atk Huge Power Tera Grass Marill Atk Cheer Tera Blast (80 BP) vs. -6 0 HP / 0 Def Tera Water Pikachu: 10778-12684 (102.1 - 120.2% of 50x hp boss) -- guaranteed OHKO
+6 252+ Atk Huge Power Tera Grass Marill Atk Cheer Tera Blast (80 BP) vs. -5 0 HP / 0 Def Tera Water Pikachu: 9474-11148 (89.8 - 105.6% of 50x hp boss) -- 37.5% chance to OHKO
+6 252+ Atk Huge Power Tera Grass Marill Atk Cheer Tera Blast (80 BP) vs. -6 0 HP / 0 Def Tera Water Pikachu in Grassy Terrain: 14012-16488 (132.8 - 156.2% of 50x hp boss) -- guaranteed OHKO
+6 252+ Atk Huge Power Tera Grass Marill Atk Cheer Tera Blast (80 BP) vs. -5 0 HP / 0 Def Tera Water Pikachu in Grassy Terrain: 12314-14490 (116.7 - 137.3% of 50x hp boss) -- guaranteed OHKO
When SV's era of support ends, I will appreciate these events for having something unique and reasonably difficult to return to the game to regularly in its lifespan, giving me the opportunity to make the entire Marill line their own champions by planning these gimmick ideas/self imposed challenges. The process of planning out and adapting the execution, all for the hope of the stars aligning and finding a path to victory for Azumarill somehow still satisfies me, perhaps because its viability ranges so wildly from being optimal to the opposite, yet still having the same core strategy. I feel like most of these 7 star raids manage to find this balance of appearing insurmountable as a blind challenge yet reasonably possible in a way that ingame battles don't get to be as tense with. Thank you game freak.