I was running some calcs and I noticed that the calc has a tiny error when tera stellar is in play.
It treats the type as its own typing (no resists or weaknesses or w/e) instead of calculating damage based on the users base typing.
Terapagos-Terastal also has its ability default to Tera Shift instead of Tera Shell
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After conducting more testing into this, it seems as though the interaction is mainly between the moves Collision Course and Electro Drift and Tera Stellar. Ordinarily, CC and ED would have an additional boost (4/3x boost according to the Smogon Dex) if their attack is super effective. For example, Miraidon's Electro Drift hitting into Kyogre will do more damage than if it used Wildbolt Storm (100 BP) into Kyogre:
0 SpA Hadron Engine Miraidon Wildbolt Storm vs. 0 HP / 0 SpD Kyogre in Electric Terrain: 360-426 (105.5 - 124.9%) -- guaranteed OHKO
0 SpA Hadron Engine Miraidon Electro Drift (133.3251953125 BP) vs. 0 HP / 0 SpD Kyogre in Electric Terrain: 480-566 (140.7 - 165.9%) -- guaranteed OHKO
However, if the Kyogre has Terastallized into a Stellar-type, then the damage calculator will not grant Electro Drift the additional boost that it would ordinarily have if Kyogre had remained a Water-type.
0 SpA Hadron Engine Miraidon Electro Drift vs. 0 HP / 0 SpD Kyogre in Electric Terrain: 360-426 (105.5 - 124.9%) -- guaranteed OHKO
0 SpA Hadron Engine Miraidon Wildbolt Storm vs. 0 HP / 0 SpD Kyogre in Electric Terrain: 360-426 (105.5 - 124.9%) -- guaranteed OHKO
The question becomes whether this behavior is accurate to cart, which is a question for
the mechanics thread.