Your team looks like a balance team, so Mega Sableye's utility is probably more beneficial than Mega Sharpedo's late-game cleaning capabilities.
Sableye @ Sablenite
Ability: Prankster
EVs: 252 HP / 112 Def / 144 SpD
Careful Nature
- Knock Off
- Will-O-Wisp
- Recover
- Foul Play
Knock Off is a good option that allows it to take away crucial items such as Chansey's Eviolite. Will-o-Wisp can burn opposing physical attackers, which is really helpful with specially defensive Tyranitar as a teammate. Recover is for regaining HP, and Foul Play punishes Pokémon that set up on Sableye.
Replace Greninja's Extrasensory with Hydro Pump. Hydro Pump has a higher base power, and it makes sense to use it if you decide to replace Sharpedo.
I think you should give Hydreigon Choice Specs instead of Expert Belt. You already have a Choice Scarf user, but giving Hydreigon a choice item isn't too big a deal considering you have multiple Pokémon that you can switch in to if you're locked into a bad move. With Expert Belt, Hydreigon misses out on a few KOs. Darkinium-Z or Dragonium-Z is also probably better than Expert Belt, as you don’t have any other Z-move users, and it provides a reliable way to KO someone.
