I’m on mobile, so I can’t give code examples, but a quick comment: you should sort your arrays and then go through them linearly in parallel, keeping them “about equal” by incrementing the index of whichever array “falls behind”. That will reduce the complexity of your solution to O(n log n) rather than O(n2).
Right, the lack of an efficient algorithm for all Arrays with Equatable elements is why this isn't there. You can do it with a Set if the elements are Hashable and by sorting copies if all elements are Comparable, but otherwise you're stuck with something inefficient, and even those two strategies require extra time and space.