This Meganium raid boss may be the hardest Azumarill solo challenge of all time, with gamefreak combining both the natural tankiness and Defense boosts of Blastoise and the Grass STAB and Growth offense of Venusaur into a mini-physical Mewtwo battle that can freely boost on both sides, which is even Psychic-type to boot. I iterated over many different sets over the hours since the raid dropped, but eventually ended up with a standout strategy that was consistently able to get past the shield and farther than anything else I tried.
Sap Sipper was considered at first, but discarded when I realized Meganium would usually just Curse for the first turns while I could Mud-Slap and Fairy tera afterwards to remove the weakness, which is also necessary to resist Body Press which isn't affected by my Attack-lowering AI, and then reconsidered again but it just didn't have enough damage, so I ended up going with Huge Power against a grass boss for once. The Azumarill set itself is fairly standard, though Play Rough PP is essential to max out and Defense is probably more useful to spec into than HP, though at this point I find it easier to recall the thresholds for 404 HP since they divide into clean numbers. In theory, I had a passing thought of a huge Rollout chain with the last hit critting but never actually tried it since it would either be off-type or weak to Body Press.
Most of my time was wasted on trying to make the Razor Claw's +1 crit stage work (1/8 ratio). It was disheartening to go for 10 hits with no crit and barely even get past the shield, and sometimes I would get past and be unable to make up the many crits I needed for the rest, feeling like I needed over half of my hits to crit. It did show me that it was possible, but I needed more critical hits to land more often, and Azumarill only has one way to do that. Once again, I had to resort to living on the edge at 25% HP to activate Lansat Berry for +2 crit (1/2 ratio), but by that point I had figured out that I could afford to take some chip damage and activate it alongside Belly Drum. I think this is more efficient time-wise though because you can give up early if you don't proc Lansat instead of just wasting all your Play Rough PP trying to crit.
I considered some cheer preservation strats while still running Razor Claw. With a defense cheer at +6, Body Press does 25 and Grassy Terrain heals 25, so survivability is pretty good at that point. However this didn't really help with the bottleneck of still needing crits and neither did the attack cheer, though it would have lowered the number of crits needed. The other problem ended up being that it wasn't reliable to intentionally set up getting killed by Meganium because I was setting up Mud-Slap at the same time and either needed Belly Drum to work off the first defense cheer or for it to choose Seed Bomb early and Staraptor to not Feather Dance too much, and also getting killed instead of using moves loses time and turns that the AI could be taking advantage of, so this usually just ended up being an incidental thing when the option came up but still didn't help the main issue of needing crits to make progress.
In terms of AI, I was fortunate enough to get Toxapex's Chilling Water, Staraptor's Intimidate/Feather Dance, and Arboliva for Grassy Terrain healing on my first group roll, though it also helped Meganium's Seed Bomb power a bit. Ideally I was thinking to get Arcanine for potential burns but decided not to risk a reroll with everyone playing a major role.
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https://stevecooktu.github.io/sv_raid_lookup/)
Looking at the boss script in detail, some small mercies Meganium provides are that it wastes 2 actions on screens that don't affect the battle at all. It also helps that the player and boss resets are completely separated, so that I can set up Mud-Slap early on and then spend my time setting up everything once after the timer. Bulldoze is a strange choice but I guess it wastes time in the endgame. Also it can double attack after 50% HP as usual.
My final strategy ended up revolving around using Mud-Slap 3 times at the start, then terastallizing to become Fairy and Mud-Slap again. Then I would wait for the shield and buff reset to trigger so I could start Defense Curling. During this process I need Meganium to chip me between 50% and 75% HP to be able to use Belly Drum and proc Lansat, which is helped by Grassy Terrain allowing me to work with higher damage and just wait to get in the right HP range while ideally dodging most other attacks. If its attack is down enough, it seems like Meganium usually goes for Body Press which is a tolerable hit, and I managed to bait it into range with an early Belly Drum after only 4 Defense Curls, going down to exactly 100 HP.
After getting to +6 Attack/Defense and critting, then all that's left to do is potentially heal up and press Play Rough and hope for crits. With Razor Claw this is a very sad affair, but with Lansat it's actually satisfying to see crits land constantly. The shield takes about 4 crits to fully take down and I usually am barely above the 70% threshold on the 3rd hit which is the first debuff reset, so I like to take a heal cheer before this point because it's still safe while Meganium has low accuracy. I decide to take the shield down first and then set a Mud-Slap afterwards, though I burst down to 40% soon after with another crit anyway.
I took a few non-crits down to 8 Play Roughs, then crit it down to both the 40% and 30% threshold for a reset and Bulldoze combo that wasn't too threatening. At this point I had plenty of time to spare and seemingly only a crit or two left to finish it, so I put up a Mud-Slap for safety and potential time saving from miss animations. I ended up watching Meganium use Curse and constantly getting barraged by Body Presses while not getting crits for 4 turns. Every time I saw the NPCs get a move off I knew the battle had not ended. Eventually we chipped it into the red as a tremendous energy gathered (10% time left/60 seconds) and I had only 3 Play Roughs left.
In the end, my 14th Play Rough struck true and landed one last crit, but Meganium decided to spite me and land in one last Body Press. I think this means an NPC actually finished it off on its turn, so I didn't get to see the text saying that it fainted on my screen. Or maybe it was the poison that did it in, because all Staraptor did was Feather Dance and Arboliva had just respawned. Either way, I don't think I will be replaying this fight for that screenshot and I hope that bosses casually spamming boosts isn't going to be the new trend of difficulty for Azumarill's sake.
Sidenote: I think the move effectiveness shown for a dead tera raid boss is based on its base type like how the overworld tera mons become their base type again when the crystals shatter, though it's easier to see this happen in multiplayer.
