Ruby and the Storm King are both childish and silly, unskilled but strong when it comes to using their powerful magic (initially in Ruby’s case), can use powerful wind attacks (Ruby uses a paper fan for hers), and are interested in magical stones (in particular, Deco Stones and the Misfortune Malachite, respectively).
 
Ruby wins. Not even a contest. SK creates a giant tornado? Ruby can just fan her way through the tornado and throw an explosive energy ball at SK to make him lose control of the tornado and it dissipate. SK blasts Ruby with lighting? She can canonically withstand that (as well as a massive explosion visible from space that only left her dazed). SK throws that last Obsidian Orb at Ruby? She can just super jump to avoid it and then roundhouse kick him to the ground. That should work just fine if the ReManeing Five can easily knock him down. SK sends his army after Ruby? She’s got a million ways to take it all out by herself (and other Jewelpets may join in so she can get back to attacking SK).