Focus Sash Cloyster can actually do a lot of work. Shell Smash once and then sweep away. And it pairs very nicely indeed with Garchomp. It's really nice for grinding since it plays FAST!
Dragonite or Salamence with Aegislash is also a pretty nice combo.
Obviously, when it comes to Water-types, my preference is for Suicune, but that's sadly absent right now. That's where we're going to need to do more experimentation. Interestingly, Dragonite / Chansey / Mega Slowbro (my most successful singles team in the Maison) is legal, but will require lots of work to obtain prebank (HA chaining, breeding Seismic Toss onto Chansey, buying a Slowbronite, possible bottle capping). Plus, it is a pretty slow team, so not the best choice for quick grinding.
Mega Scizor (or even regular Scizor, if you use another Mega) is also a staple, so that's another fine option. In short, it seems like you are already on the right path!