Volcarona (F) @ any viable volc item
Ability: Flame Body
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Quiver Dance
- Fiery Dance/Fire Blast if running Firium Z
- Giga Drain
- Hidden Power [Ground]/Psychic if running Psychium Z
Diancie-Mega @ Diancite
Ability: Magic Bounce
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 1 Atk
- Power Gem
- Moonblast
- Hidden Power [Fire/Ice]
- Earth Power
A core I've dubbed "Deadly Divas", Volcarona and Mega Diancie have surprisingly good type sync. Volcarona pwns the Steel, Water (assuming you've set up QD and are running Giga Drain), Ground (see Water), and Grass types that make M-Diancie's life miserable, while she returns the favor by KOing the Rock, Flying, and Dragon types that prevent Volcarona's rise to Ubers (Water types not named Gyarados/Empoleon get heavily dented/OHKOed by Giga Drain so they are a non-issue) plus both excel in the sweeping department. Chansey counters are a must as that fat pink blob stops this core cold, but other than that, I would also strongly recommend a Lando-T and Gren (both formes) check as they (especially Gren) eat this core for breakfast (although no gren wants to come in on giga drain and sets without Water Shuriken are a non-issue for Volc and Lando survives anything from either mon save Diancie's HP Ice and OHKOes both in return with Edge or EQ respectively)
tldr; volc and m-diancie work wonders together assuming chansey is dead/a non-issue, use them