Ok, let's start with Mewtwo. Mewtwo is bothered mainly by physical sweepers such as Scizor and Metagross, but also is annoyed by Blissey (technically, the Taunt/CM set beats it IIRC). But to lure in Blissey, we can try a Physical Mix Palkia.
Now we need to support the Palkia, so we use a Mono-CM Kyogre, which checks Groudon too, and can also beat Mewtwo.
You don't necessarily need a dedicated physical sweeper, but you DO need a way to get past Blissey. A physical sweeper won't necessarily cut it, since a Blissey player will just run away to a Groudon or Giratina.
We need 3 more Pokemon. I notice that we're fairly weak to Scarf Zekrom, so I was thinking that we could probably use Groudon to check it and set up SR while avoiding being setup fodder.
At this point, I'd probably tack on a lead that could limit Deo-S to one layer. Maybe Scarf Darkrai. That would also check many other threats, but be a bit risky. Then, play around with Pokemon for the last slot.