Tyranitar was actually a pretty close solo for Azumarill. The 600 BST pseudo stat gap was very tight to make up for and I needed to squeeze out every advantage I could get, aside from taking advantage of the Ghost-type matchup which isn't very relevant for either side otherwise, but I still stand by using all-natural grass fed tera types.
I actually had to adjust my EVs for this one, which I don't do often because this is my non-optimized playthrough Azumarill. It would be more optimal to fully invest in Defense or something and not be Jolly but Azumarill is a jolly fellow. I was actually missing 1 EV from having max Attack and shifted some over to Defense, which slightly lessens the risk of getting 2HKO'd by crit Stone Edge but is still very real. Azumarill needs 159 speed to outspeed Tyranitar at neutral. I was relying on Mud Shot Bellibolt to help me outspeed, but hitting it under 40% HP to debuff reset ended up with me getting hit twice in a row and ending a run, so I decided to avoid that risk entirely, though in the end having rain up allowed me to hit to under 40% HP from the shield break turn anyway so I don't know if it's totally necessary. But it does help out setup in the midgame as well.
The moves are typical Azumarill solo things at this point with Mud-Slap to build tera/dodge everything, Belly Drum to set up, and STAB Liquidation to do damage, though with most other stat boosting/debuffing moves being fairly useless, Rain Dance fills in the last slot and serves a dual purpose of cancelling out Sand Stream and pumping up STAB Liquidation's damage output which is actually pretty relevant.
Speaking of good AI, I locked in Bellibolt/Toxapex/Staraptor, hoping for Gardevoir for healing but settling for at least attack and speed control. Despite the guaranteed crits, Attack debuffs from Intimidate and potentially Chilling Water do help to mitigate Tyranitar from getting any stronger than neutral from Dragon Dance. Bellibolt seems to go for Mud Shot which can at least help cancel out Dragon Dance to let Azumarill outspeed, but otherwise does useless Light Screens and doesn't seem to Discharge.
One thing you do have to watch for is waiting until Staraptor actively respawns and activates Intimidate to cancel out Dragon Dances before actually inputting a move, because if it's still fainted and not present on the field then it hasn't activated yet. The chance of Staraptor fainting to proc Intimidate is also not perfect due to Stone Edge's accuracy, but there's nothing to be done about that.
Azumarill's general strategy to start off is to set Rain Dance T1 (mainly to get rid of the rude sandstorm), then set an attack cheer and hopefully faint on T2 or T3 by getting hit twice to have both cheers permanently active for the rest of the fight, as well as burn time for the resets and shield to go through before starting the fight. (I used 1 PP on Belly Drum on the turn I died.)
Tyranitar is basically guaranteed to 2HKO if it hits since the AI defense cheer isn't active on T1, and if Azumarill is low enough from turn 1 then it seems to go for a better accuracy move like Shadow Claw. After the defense cheer is active, Tyranitar has about a coinflip chance to 2HKO/3HKO Azumarill with critical Stone Edges, though with my EVs it's more:
0+ Atk Tera Ghost Tyranitar Stone Edge vs. 84 HP / 88 Def Tera Water Azumarill through Def Cheer on a critical hit: 165-195 (45.5 - 53.8%) -- 41% chance to 2HKO
From there, Azumarill just has to Mud-Slap 3 times and set up to tera as usual, though I actually went for 4 Mud-Slaps for safety while setting up Belly Drum and then rain right after to get greedy and keep it up for as long as possible. I could and did take an extra death in this phase as well and still barely make it on time, but rain and Liquidation defense debuffs definitely help the time pressure in that case. It is fortunate that Tyranitar seems to prefer the inaccurate Stone Edge overall from full HP.
With a permanent attack cheer Tera Water Liquidation under rain, Azumarill 3HKOs the shield and heals up from Shell Bell about evenly against critical Stone Edge hits, even after the debuff reset, which is good because there is very little time to press anything other than Liquidation at that point. A defense debuff on the second-last turn/5th Liquidation made up for damage with the rain ending at the same time, and it was pretty much down to the wire even with that bit of luck.
+6 252 Atk Huge Power Tera Water Azumarill Atk Cheer Liquidation vs. 0 HP / 0 Def Tera Ghost Tyranitar in Rain: 1702-2004 (16.6 - 19.5% of 30x hp boss) -- possible 6HKO
Overall a pretty raw fight that still just comes down to the basics, but did make me spec to outspeed it and pull out cheer preservation plus Rain to finish in time, which is more than I can say of a lot of bosses from memory. Now I'm kind of scared for Clear Body Metagross, but we'll burn that bridge when it comes to it.
Then got crit'ed by Earthquake before Tera-ing and Power Tripping to victory. Be careful after you tera, Stone Edge crit does just over half your HP, though the +6 Tera Dark Power Trip and Shell Bell should be enough for a full heal.
