Seen 2 Hours Ago
Posted 1 Day Ago
609 posts
5.3 Years
I think that you'll be best off treating your Pokémon as if it had an ability that doubled type-effectiveness (e.g. look at how Solid Rock etc work) and gives it more powerful STAB (look at Adaptability). You don't actually have to create an ability for this—just check for the species in the places where you'd normally check for an ability.

Also, I don't think it's super relevant but if you actually managed to give it two types that worked in the obvious way (i.e. all computations involving type run twice), double-STAB would be 2.25× because it's 1.5× applied twice.