A record 55x HP multiplier (for 20405 raw HP before shield reduction) and several resets makes for quite a behemoth of a raid boss, even with Glaive Rush to theoretically cut that down. I couldn't really tell if it went for it or not, mostly because I was Azumarill, but still. I imagine it would have to go for it on a NPC to count in solo at least, and that might be a hard sell before the random double attack phase.
After seeing how little damage my tera Liquidation did on a test run, and then seeing the full script and imagining having to re-set up past the several resets, instead of timing all that, I decided to just go with the old reliable strategy of brute force with a big super-effective Rollout solo.
The set is the typical tera Rock, Shell Bell, Huge Power max HP/Attack Defense Curl/Rollout/Belly Drum/Mud-Slap Azumarill. Baxcalibur is pretty vulnerable to debuffs after 50% time, and Mud-Slap is my debuff of choice to save time and HP though Chilling Water could work just as well.
AI notes: Staraptor helped to cancel out the initial Swords Dance and keep it busy without running Chilling Water myself. Tauros works too but not Arcanine. Garganacl seemed to use Rock Slide more than the potential Rock Tomb for speed control, but it did get to -1 speed by the end. Gardevoir is generous as always with Life Dews.
My opening was a Belly Drum, aiming to faint within the first 3 turns to set an offense cheer and also preserve the AI defense cheer. +1 Icicle Crash serves well enough to put you in death range in the next turn but sometimes it can flinch, I imagine it will kill within 3 turns even after a defense cheer though.
+1 0+ Atk Tera Ice Baxcalibur Icicle Crash vs. 252 HP / 0 Def Azumarill: 167-197 (41.3 - 48.7%) -- guaranteed 3HKO
After having both cheers permanent, I burned the remaining time before the 50% debuff reset by setting up Defense Curls to +6 and also Belly Drum to save time later. As usual, Defense Curl is vital for doubling Rollout's power off the bat, but also helps out since Baxcalibur's debuff reset will also remove the Intimidated stat drops later, and there's nothing really better to do since Mud-Slap
Then I got 3 Mud-Slaps in (I think I got flinched once between them) and decided to send my tera Rollout from there and watch the boss go down.
Though it missed immediately on the first turn, so I had to send it again.
The shield went down in 3 Rollouts, though this procced the buff reset on 60% HP.
Still even without Belly Drum or Huge Power active, Rollout 4 hurt quite a bit and there was still a 5th Rollout to spare.
Though this did proc the 40% HP buff reset, so once again I didn't have Huge Power active for a turn. And also Baxcalibur got a Swords Dance off.
Icicle Crash kind of hurt and could have flinched, but then I landed Rollout 5 and it didn't actually finish it off, but I healed to full anyway.
So in the end, I had to add another input to finish it off with a simple first hit of Rollout and hope it worked out, though it at least gave me an opportunity for some nice finishing shots of the status screens.
It hit me with Earthquake, doing about half again but forgoing a flinch chance.
Finally, the light Rollout finished the job.
Calcs:
+6 252+ Atk Huge Power Tera Rock Azumarill Atk Cheer Rollout (60 BP) vs. 0 HP / 0 Def Tera Ice Baxcalibur: 1530-1802 (7.4 - 8.8% of 55x hp boss) -- possibly the worst move ever
+6 252+ Atk Huge Power Tera Rock Azumarill Atk Cheer Rollout (120 BP) vs. 0 HP / 0 Def Tera Ice Baxcalibur: 3060-3600 (14.9 - 17.6% of 55x hp boss) -- possible 6HKO
+6 252+ Atk Huge Power Tera Rock Azumarill Atk Cheer Rollout (240 BP) vs. 0 HP / 0 Def Tera Ice Baxcalibur: 6116-7196 (29.9 - 35.2% of 55x hp boss) -- 23.6% chance to 3HKO
252+ Atk Tera Rock Azumarill Atk Cheer Rollout (480 BP) vs. 0 HP / 0 Def Tera Ice Baxcalibur: 1530-1802 (7.4 - 8.8% of 55x hp boss) -- possibly the worst move ever
252+ Atk Tera Rock Azumarill Atk Cheer Rollout (960 BP) vs. 0 HP / 0 Def Tera Ice Baxcalibur: 3060-3600 (14.9 - 17.6% of 55x hp boss) -- possible 6HKO
Judging from these calcs, I might have gotten some Glaive Rush procs on the buffless Rollout turns, maybe? I feel like they did about double that of what was expected on those turns. Either that or Huge Power came back mid-turn.
Either way I'm perfectly fine with taking a quick and satisfying win regardless of knowing how it happened, and a boss that could take a full barrage of super effective tera permacheered Rollouts is respectable enough to acknowledge, even if it was more due to having 2 late resets.
Rollout is fun in that way in that you can just rely on the power of numbers doubling enough and staying there even if your other visible stat stages get reset, and I'm happy to see it carry me through this without too much trouble.
EDIT:
I also had a nice attempt with the perfect 2 Intimidators originally, but ran out of time right before Rollout 5 would've gone through.