Get a furret and trade to get a castform prefer + speed nature and some speed ev training on bats in the rail, lead with a heat rock castform ( If you have lot of fire pokemon) or a Telluric Seed castform (one turn safety) and a Growlithe with flamethrower. The idea situation start is illumise use raindance due to her prankster, castform use sunny day turn to rainbow field, Growlithe use flamethrower to kill illumise to prevent her second raindance. Then you can enjoy the over power rainbow boosted weather ball. Her second raining dance pokemon is araquanid, it is slow and not so much tanky. For backup if you get rockuff or drilbur from mystery egg, they are great with rock slide and are future proved, or just some other decent flying/rock pokemon. Some of my person recommendation that carried my for a long time and you can get before shelly are sturdy Donphan strong physical tank with not bad damage, Zebstrika he have flame charge to deal with shelly and strong physical electric type with decent speed, and Galvantula not that much about shelly it just fast and can learn sticky web.