All Megas up to this point have a fairly drastic change one way or another for some sort of niche. Just throwing stats at Sableye isn't going to do anything significant in its current incarnation. The ability would certainly be one way to do it.
Like a user mentioned earlier, Wonder Guard isn't entirely out of the question if a typing change is involved, which Rock type seems a pretty safe bet if Diancie is anything to go by. Dark/Rock or Rock/Ghost, pick your poison.
I could easily see them doing some sort of whacko spread like 50/165/35/155/25/50 to make the instances it does get hit, pretty much a K.O, all while being a sort of Aegislash behind the comfy "shield" of Wonder Guard. Reducing the defenses may not make sense at a glance, but keep in mind this guy seems awfully keen on hiding behind that giant rupee for whatever reason.
Even then, with all of the Mold Breaker going around lately, I just don't see why this is a big deal as it's being made out to be. I agree that it would be an issue if the type stayed the same, which correct me if I'm wrong, there's no confirmation of it being so.
Anyway, that's just how I'd go about with it, lol.