I went with Volcarona to solo this. Basically the Delphox that was mentioned earlier, but without needing a former event/many league points.
Volcarona
@ Shell Bell
Tera Fire
252+ SpA, 72 SpD, 184 Speed
Fiery Dance
Struggle Bug
Quiver Dance
Sunny Day
Turn 1 Sunny Day, Turn 2 Struggle Bug, Turn 3 Quiver Dance, Turn 4 and 5 Struggle Bug, Turn 6 Sunny Day*, then Tera-Fiery Dance until the reset.
Once Blastoise gets its first reset, I made a convenience Struggle Bug that was not really needed.
When I got resetted, I used Quiver Dance to outspeed Blastoise after Shell Smash. In theory. In practice, I forgot to hyper-train my Pokémon, so I used two Quiver Dances instead.
Then sun-boosted Fiery Dances until the end.
I was afraid about Hydro Pump doing a lot of damages, and it turned out I tanked better than expected. Even without the Hyper Training. Even with Blastoise hitting every single Hydro Pump every single turn (it didn't use any other move).
* Unless you have 30 HP or less. Then you need the recovery from Tera-Fiery Dance first. Should not happen unless extreme bad luck.