Starting the battle at 1 HP, Koko gets the job done
Tapu Koko @ Electric Seed
Ability: Electric Surge
EVs: 220 HP / 252 Def / 36 Spe
Timid Nature
IVs: 0 Atk
- Taunt
- Roost
- Protect
- Toxic
Protect turn 1. Roost if they Moonblast, Taunt if they don't.
0 SpA Cresselia Moonblast vs. 220 HP / 0 SpD Tapu Koko: 68-81 (20.2 - 24.1%) -- guaranteed 5HKO
Roost->Taunt->Toxic, Roost etc
176 Atk Zapdos-Galar Brave Bird vs. +1 220 HP / 252+ Def Tapu Koko: 48-57 (14.2 - 16.9%) -- possible 6HKO

Roost until you can live an Earthquake->Toxic->Alternate Protect and Roost. Tested this and they land on average 7 Earthquakes in a match which isn't enough to crit most of the time. Some of those Earthquakes are also non-lethal.
124+ Atk Incineroar Earthquake vs. +1 220 HP / 252 Def Tapu Koko: 116-138 (34.5 - 41%) -- guaranteed 3HKO
Tapu Koko @ Electric Seed
Ability: Electric Surge
EVs: 220 HP / 252 Def / 36 Spe
Timid Nature
IVs: 0 Atk
- Taunt
- Roost
- Protect
- Toxic

0 SpA Cresselia Moonblast vs. 220 HP / 0 SpD Tapu Koko: 68-81 (20.2 - 24.1%) -- guaranteed 5HKO

176 Atk Zapdos-Galar Brave Bird vs. +1 220 HP / 252+ Def Tapu Koko: 48-57 (14.2 - 16.9%) -- possible 6HKO

Roost until you can live an Earthquake->Toxic->Alternate Protect and Roost. Tested this and they land on average 7 Earthquakes in a match which isn't enough to crit most of the time. Some of those Earthquakes are also non-lethal.
124+ Atk Incineroar Earthquake vs. +1 220 HP / 252 Def Tapu Koko: 116-138 (34.5 - 41%) -- guaranteed 3HKO