Diancie seemed like something that synergizes Cresselia a fair lot, as Cress is happy to switch in on Ground types that threaten Diancie and it tends to invite Incineroar on the field, which Diancie is happy to switch into and DStorm down. Diancie also sets broken Stealth Rocks and scares your opponent from setting them thanks to Magic Bounce, which helps a lot in slow slugfests that tend to happen in games with a Cresselia involved.
Diancie also appreciates Incineroar support a lot, Fake Out gives it turns to get started with either the SR or the DStorm clickage, Intimidate is a good ability, Flare Blitz smashes Steel types that threaten it, and Diancie's ability to deter the opponent from clicking SR freely makes Incineroar even better.
I picked Celesteela as my Steel type because once again it invites Incineroar on the field which is good for Diancie, and it answers Mega Metagross very reliably which otherwise troubles Diancie a lot. Celesteela is also handy in answering Grass-type attacks coming from the likes of Kartana, Lurantis, Tapu Bulu, etc. which Diancie is scared of
At this point I have a 4-mon core with massive issues against Water and Electric types and a lot of slowness, so I opted to commit to semi-room and add Gastrodon which takes care of both of those weakness quite reliably, and already has his Grass weakness covered.
To round out the team I added Tapu Koko which offers extra offensive coverage against Water-types, another fast option outside of Trick Room which helps me control the pace of the game a lot, as well as terrain control where necessary. (and a second fairy to block the clanging)
As for set choices, I was set out on a support Cress set from the start because I don't like how Toxic completely shuts down the CM set; TR is an easy choice for speed control given the rest of the team, Toxic helps with taking down bulkier threats which I don't have many options for otherwise, Moonlight gives much needed longevity and Psychic is its most reliable attacking move for only one slot, but I guess Ice Beam is also an option since this team relies on Cress a fair lot to deal with grounds.
Diancie and Steela are both straightforward and standard set choices really, rocks are broken I'm never running earth power
Koko I wanted to be AV to be happier when switching into strong special Water moves (cases where my Gastrodon is dead or other sticky situations like that), I don't really need it to nuke things from full anyway since the rest of the team is great at getting chip down. HP Fire is not really necessary since Celesteela and Incineroar already snipe steel types really well, so I opted for double electric + Gleam + NM, but Sky Drop probably isn't bad either
Incineroar drops Knock Off for Taunt to cover Trick Room match ups as well as various annoyances like Amoonguss, Flare Blitz is really helpful in pressuring Steels out of the field which in turn frees up Diancie to go wild so I didn't want to drop it, Fake Out is a broken move, and the slow U-Turn is also quite nice for general pivoting purposes.
Gastrodon is left and it can make good use of a Z-Crystal so the choice was pretty straightforward there as well.
This team struggles a bit with fire types not named Incineroar, especially ones that run grass moves for Gastrodon (CharY, Torkoal), but if you can keep the pressure on them with Diancie it should be fine