The most important thing is not to deal heavy damage to Tapu Koko, but to keep it from styling all over your team with fast Electric Terrain boosted STAB moves like Thunderbolt, Volt Switch or Gigavolt Havoc.
4 HP / 28 SpD Thundurus-T can switch into anything Tapu Koko could throw at it and even survive Choice Specs Hidden Power Ice or Dazzling Gleam while Stealth Rock is active.
Volt Absorb keeps opponents from blindly spamming Electric-type moves in Electric Terrain, which makes it much harder for them to deal significant damage with Thunderbolt or grab momentum with Volt Switch.
In return Thundurus-T can try to steal the momentum with his own Volt Switch or just go for the kill with a super effective Sludge Wave.
Calm Mind Life Orb Tapu Koko also misses the OHKO against a healthy Thundurus-T from +1, while being outsped and 2HKO'ed by Sludge Wave. The Choice Scarf is important to gain that Speed advantage.
If you should (unknowingly) switch into a Choice Scarf Tapu Koko, it still only has a miniscule 0.4% chance to 2HKO a healthy Thundurus-T with Hidden Power Ice before you OHKO it with a Sludge Wave.
Thundurus-Therian @ Choice Scarf
Ability: Volt Absorb
EVs: 4 HP / 4 Def / 220 SpA / 28 SpD / 252 Spe
Timid Nature
- Thunderbolt
- Volt Switch
- Sludge Wave
- Hidden Power [Ice]
Offensive Calcs
220 SpA Thundurus-Therian Sludge Wave vs. 4 HP / 0 SpD Tapu Koko: 146-172 (100 - 117.8%) -- guaranteed OHKO
220 SpA Thundurus-Therian Sludge Wave vs. +1 4 HP / 0 SpD Tapu Koko: 98-116 (67.1 - 79.4%) -- guaranteed 2HKO -- (4 HP Calm Mind Tapu Koko is a waste of stat points, but you never know...)
220 SpA Thundurus-Therian Sludge Wave vs. 0 HP / 4 SpD Tapu Koko: 146-172 (100.6 - 118.6%) -- guaranteed OHKO
220 SpA Thundurus-Therian Sludge Wave vs. +1 0 HP / 4 SpD Tapu Koko: 98-116 (67.5 - 80%) -- guaranteed 2HKO
220 SpA Thundurus-Therian Volt Switch vs. 4 HP / 0 SpD Tapu Koko: 40-48 (27.3 - 32.8%) -- guaranteed 4HKO
220 SpA Thundurus-Therian Volt Switch vs. 0 HP / 4 SpD Tapu Koko: 39-47 (26.8 - 32.4%) -- guaranteed 4HKO
(Relevant Volt Switch calculations in case of a predicted overpredict that got predicted by a predicting opponent capable of prediction who saw the Volt Switch coming and stayed in with his Tapu Koko.)
Defensive Calcs
252 SpA Choice Specs Tapu Koko Dazzling Gleam vs. 4 HP / 28 SpD Thundurus-Therian: 96-114 (61.9 - 73.5%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Tapu Koko Hidden Power Ice vs. 4 HP / 28 SpD Thundurus-Therian: 96-114 (61.9 - 73.5%) -- guaranteed 2HKO after Stealth Rock
+1 252 SpA Life Orb Tapu Koko Dazzling Gleam vs. 4 HP / 28 SpD Thundurus-Therian: 125-148 (80.6 - 95.4%) -- guaranteed 2HKO
+1 252 SpA Life Orb Tapu Koko Hidden Power Ice vs. 4 HP / 28 SpD Thundurus-Therian: 125-148 (80.6 - 95.4%) -- guaranteed 2HKO
252 SpA Choice Scarf Tapu Koko Dazzling Gleam vs. 4 HP / 28 SpD Thundurus-Therian: 64-76 (41.2 - 49%) -- guaranteed 3HKO
252 SpA Choice Scarf Tapu Koko Hidden Power Ice vs. 4 HP / 28 SpD Thundurus-Therian: 66-78 (42.5 - 50.3%) -- 0.4% chance to 2HKO
252 SpA Fairium Z Tapu Koko Twinkle Tackle (160 BP) vs. 4 HP / 28 SpD Thundurus-Therian: 127-151 (81.9 - 97.4%) -- guaranteed 2HKO
PS: I hate Focus Blast with a burning passion but an argument could be made for it on this moveset. Still, I refuse to list it. :P
EDIT: I moved 16 EVs from Special Attack to Special Defense, because 220 Special Attack EVs are enough to OHKO all Tapu Koko that only invest 4 EVs into bulk.
The extra Special Defense significantly reduces the chance of Choice Scarf Tapu Koko achieving a 2HKO with Hidden Power Ice (5.9% > 0.4%).
Not sure if this messes up any other important damage rolls for Thundurus-T, but this set is about handling Tapu Koko so that's that.
Change the EV spread if you want or need to, though I wouldn't go below 4 HP / 12 SpD bulk. Otherwise you risk getting OHKO'ed by Choice Specs Tapu Koko after Stealth Rock damage,
which defeats the purpose of this Thundurus-T.