What are some good partners for RP Diancie?
Hazards are important to help Diancie net the KOs it needs on a sweep with its high-but-not-monstrous power. Ferrothorn and Skarmory can lay both types of hazards depending on what your team benefits more from, and offer defensive synergy vs most Steels/Bullet Punch Pokemon, while Heatran can be a more offensive SR setter while also matching up well with most Fire, Steel, and Grass Types, though watch the shared Ground weakness. You'll also definitely need some way to deal with Mega Venusaur, which comfortably takes even the rare Psyshock, KO's with Giga Drain (even Defensive 2HKO's), and also can use different moves to lure the suggested partners (EQ or HP Fire)
252 SpA Mega Diancie Psyshock vs. 232 HP / 0 Def Mega Venusaur: 170-202 (47.3 - 56.2%) -- 78.5% chance to 2HKO
252+ SpA Mega Venusaur Hidden Power Fire vs. 252 HP / 4 SpD Skarmory: 184-218 (55 - 65.2%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Mega Venusaur Hidden Power Fire vs. 252 HP / 168 SpD Ferrothorn: 212-252 (60.2 - 71.5%) -- guaranteed 2HKO after Leftovers recovery
0- Atk Mega Venusaur Earthquake vs. 248 HP / 0 Def Heatran: 248-292 (64.4 - 75.8%) -- guaranteed 2HKO after Leftovers recovery (Defensive set just used to show Heatran always loses to EQ).
While Diancie matches well against Mega Sableye, you'll probably also need something prepared to break Stall/Defensive cores. Kyu-B deals well with Mega Venusaur, and depending on the overall set has the potential to deal with Heatran, SpD Gliscor and Hippowdon to name a few, while also resisting Grass and Water while Diancie beats Fighting Types and is Immune to Dragon (though beware the compounded Steel Weakness)