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This is what I'm thinking as well.
I'm not sure how your battle video disproves this theory, unless I'm missing something. All of those sets and pokemon are available currently, no?
I'm not using the one on showdown, I'm using this one: http://turskain.github.io/
If assault vest was being calc'd it would look like this:
0 SpA Slowbro-Mega Psychic vs. 252 HP / 0- SpD Assault Vest Magmortar: 51-60 (28 - 32.9%) -- guaranteed 4HKO
I believe I found an error with the calculator, either that or I'm doing something wrong.
Here is what the calc with the battle tree sets gives me:
0 SpA Slowbro-Mega Scald vs. 252 HP / 0- SpD Magmortar: 66-78 (36.2 - 42.8%) -- guaranteed 3HKO
0 SpA Slowbro-Mega Psychic vs. 252 HP / 0- SpD...