I started building with diancie then I added Volcanion to prevent Steel-types from switching into Diancie's attacks and to have a better mu vs rain which diancie dislikes a lot and which is kinda popular right now. After that, Landorus helps a lot here being a ground immunity, checking Tapu Koko and Snorlax and giving Intimidate support and luring some defensive mons that could stop Diancie such as ferrothorn or tapu fini with its Z-Move. Marshadow was added later to help Diancie's work at breaking opposing team, having really good offensive synergy together. Lastly, Zapdos was added for speed control, and Ferrothorn was added as a secondary check to rain, pivot, Psyspam check and win-condition.