At the Terajuma island, there are rocks you can mine with the kit. If you want (and what i did) was to keep mining the rock until something i want appeared, soft resetting if i didn't. So that's a way to get the sun stone? for the helipotile to become a heliolisk.
I think that electric types are the way to go. The field boosts electric moves by a bit and most her mons carry ice beam for the grass types. manectric, Zebstrika and heliolisk (preferably with dry skin) should be able to do some work against her, and if necessary bring in a grass type for any ground/water or electric/water types she has.