Although I can understand that Mega Sableye can seem extremely overwhelming, there are many more Pokemon this generation that can handle Mega Sableye than previously, however with the addition of Toxapex to the defensive core, it will definitely be on the watchlist as the meta begins to take shape through the first week, so stay tuned.
After further testing, I want to address this post. I can confirm that there
are few (not many) new mons that can handle Mega Sableye currently, and
it's not even guaranteed. Let's address one by one.
Note that these are new mons, so Clefable, Xern, doesn't count.
Buzzwole: This is probably the best new mon to handle Msab right now, and it only works when Sab isn't in Mega form or Buzz isn't switching in (due to Will-o-wisp/Toxic) or Msab isn't carrying Clam Mind + Shadow Ball. It's the best new answer and yet it's not guaranteed.
Tapu Bulu: See above. It's the same as Buzzwole but
you're giving Sableye passive recovery. And also
, Toxapex handles this, so you'd better use Buzzwole than this.
Tapu Lele: Any non-CM variant gets Metal Bursted and 120 HP Msab with standard spread can easily live a Moonblast (it does around 50% - 60%). After a CM, Msab vs non-LO variants live a Moonblast and can Metal burst back. Oh, if you think you can boost CMs, Knock Off does a ton (40% - 48%) and will remove your LO.
Xurkitree: Before tail Glow, Tbolt doesn't do much damage. After tail Glow, it doesn't 1HKO (does 70% - 80%). You know what this means? Metal Burst. And this mon isn't even good in GotG right now. Edit: Oh, after you see the calc, CM sab also handles this. In my opinion, this is not a good answer.
You see, Mega Sableye in GotG is broken because it can pretty much 1v1 90%+ mon in the meta. Think of Dragonite in metas like Hidden Type or Mix and Mega, which can basically beat any mon.