I'd go Magnezone. Assuming you have passed the Ground Trial, it's really good since it has a resistance to the Dragon Trial, a super effective STAB to the Fairy Trial, as well as resistance. It also has super effective STAB to the Flying E4, as well as being resistant to a bunch of the E4 STAB, like Olivia. Also, if you picked Incineroar, against Hau's Vaporeon, and resistant to a bunch of his STAB Moves. It also has really good Stats and Moves, I myself used one up until I got Solgaleo in my Sun Game. Ooh, and I forgot, since your right there in Vast Poni Canyon, that's the electric field for Magnezone.
Source: Experience, and the Magnezone page, and the E4 USUM Serebii page.