The first of the two Pokemon we built around; this set did not get changed throughout the process of the CCAT and provides status absorption, an electric immunity, and a way to clean unprepared teams.
The other member of the core. Without Swampert as a SR setter we need Empoleon, it also provides a grass immunity, hazard clear, and can phase out threats that seek to set up on it.
The given core was supported well by a strong wallbreaker, and in this regard very little can beat Specs Keldeo for Water. We have triple STAB + HP Bug, to hit Celebi which otherwise does a lot of damage to our team.
From here it was obvious we needed a Mega Venusaur check, as well as Grass-types in general. The best option here was DD Gyarados to give us sweeping potential as well as ways to break through physical walls for Swampert.
At this point we needed a Physical wall to complement Empoleon, Slowbro was chosen to pivot around as well as spreading Toxic to whittle down the walls that this team struggles with.
At this point we had a lot of options for the last mon, we decided that Cloyster was a good blanket check to a lot of things as it sweeps a lot of unprepared teams and offers some form of speed control with sash intact.