Arceus-Dark @ Dread Plate
Ability: Multitype
Tera Type: Dark
EVs: 252 SpA / 172 SpD / 84 Spe
Modest Nature
IVs: 0 Atk
- Acid Spray
- Calm Mind / Light Screen / Snarl
- Recover
- Judgment
I’m intrigued about Arceus-Dark. Yes, it has a Fighting-weakness, but you have 120/120 special bulk, and access to ways to boost your SpDef, not to mention other teammates that’ll be running debuff moves, as well:
0+ SpA Tera Psychic Mewtwo Focus Blast vs. +1 0 HP / 172 SpD Arceus-Dark: 136-162 (35.6 - 42.5%) -- guaranteed 3HKO
0+ SpA Tera Psychic Mewtwo Aura Sphere vs. +1 0 HP / 172 SpD Arceus-Dark: 92-110 (24.1 - 28.8%) -- 98.4% chance to 4HKO
252+ SpA Dread Plate Tera Dark Arceus-Dark Judgment vs. -6 0 HP / 0 SpD Tera Psychic Mewtwo: 2364-2784 (19.1 - 22.5% of 35x hp boss) -- possible 5HKO
Personally, I think it’ll opt to run Aura Sphere over Focus Blast since it’s way more reliable, on-brand, and can’t be stopped by accuracy debuffs like Mud-Slap, but even Focus Blast can be halted with Arceus’s wide range of support moves.
You can go for a self-sufficient offensive set with CM, or a more team-supporting build with Light Screen or Snarl. Either way, you’ll be doing tremendous damage with Acid Spray and Tera Dark-boosted, Dread Plate-boosted Judgment. This allows Arceus-Dark to play an offensive support role. You could make a case for Arceus-Ghost or Bug to offset the Fighting weakness, but it’ll opt to use (potentially Psyshic Terrain-boosted) Psystrike vs you, which you can’t stop with Calm Mind.
That being said, if Mewtwo opens the raid (t0) with Nasty Plot AND it has Focus Blast, you’re kinda screwed without heavy support, which is why I also have an alternative:
Arceus-Psychic @ Mind Plate
Ability: Multitype
Tera Type: Dark
EVs: 112 HP / 60 SpA / 252 SpD / 84 Spe
Modest Nature
IVs: 0 Atk
- Acid Spray
- Calm Mind
- Dark Pulse
- Recover
A safer, but not as powerful way to go about it would be to set up with Arceus-Psychic, then Tera Dark when the time is right. You resist Psychic and Fighting, and Shadow Ball bounces off compared to Focus Blast:
0+ SpA Tera Psychic Mewtwo Shadow Ball vs. +1 112 HP / 252 SpD Arceus-Psychic: 88-104 (21.5 - 25.4%) -- 0.1% chance to 4HKO
+2 0+ SpA Tera Psychic Mewtwo Shadow Ball vs. +1 112 HP / 252 SpD Arceus-Psychic: 172-204 (42 - 49.8%) -- guaranteed 3HKO
Arceus-Psychic @ Mind Plate
Ability: Multitype
Tera Type: Psychic
EVs: 172 HP / 252 SpD / 84 Spe
Calm Nature
IVs: 0 Atk
- Acid Spray
- Light Screen
- Snarl
- Recover
You could also go for a pure support build that combines phenomenal utility and longevity with Recover.