Behold!

Camerupt @ Assault Vest
Ability: Solid Rock
Tera Type: Rock
EVs: 32 HP / 252 SpA / 224 Spe
Modest Nature
IVs: 0 Atk
- Earth Power
- Fire Blast
- Overheat
- Tera Blast
Given EVs to outspeed Mega Mawile. Even with tera, Tapu Koko can't 2HKO Camerupt with crits.
Calcs:
252+ SpA Camerupt Earth Power vs. 116 HP / 0 SpD Mawile-Mega: 290-344 (107.4 - 127.4%) -- guaranteed OHKO
252+ SpA Camerupt Earth Power vs. 0 HP / 4 SpD Tapu Koko: 354-416 (125.9 - 148%) -- guaranteed OHKO
252 SpA Tera Ice Tapu Koko Hidden Power Ice vs. 32 HP / 0 SpD Assault Vest Camerupt: 67-81 (23.1 - 28%) -- 82.1% chance to 4HKO
252+ SpA Camerupt Fire Blast vs. 0 HP / 4 SpD Tera Ice Tapu Koko: 428-506 (152.3 - 180%) -- guaranteed OHKO
252+ SpA Camerupt Earth Power vs. 0 HP / 4 SpD Tapu Koko: 354-416 (125.9 - 148%) -- guaranteed OHKO

Camerupt @ Assault Vest
Ability: Solid Rock
Tera Type: Rock
EVs: 32 HP / 252 SpA / 224 Spe
Modest Nature
IVs: 0 Atk
- Earth Power
- Fire Blast
- Overheat
- Tera Blast
Given EVs to outspeed Mega Mawile. Even with tera, Tapu Koko can't 2HKO Camerupt with crits.
Calcs:
252+ SpA Camerupt Earth Power vs. 116 HP / 0 SpD Mawile-Mega: 290-344 (107.4 - 127.4%) -- guaranteed OHKO
252+ SpA Camerupt Earth Power vs. 0 HP / 4 SpD Tapu Koko: 354-416 (125.9 - 148%) -- guaranteed OHKO
252 SpA Tera Ice Tapu Koko Hidden Power Ice vs. 32 HP / 0 SpD Assault Vest Camerupt: 67-81 (23.1 - 28%) -- 82.1% chance to 4HKO
252+ SpA Camerupt Fire Blast vs. 0 HP / 4 SpD Tera Ice Tapu Koko: 428-506 (152.3 - 180%) -- guaranteed OHKO
252+ SpA Camerupt Earth Power vs. 0 HP / 4 SpD Tapu Koko: 354-416 (125.9 - 148%) -- guaranteed OHKO