Counter
Cresselia @ Leftovers
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Moonlight
- Psychic
- Ice Beam
- Light Screen / Reflect
Cress is a very solid answer to Mega Diancie, and a lot of new Megas overall. It can take any of Diancie's attacks comfortably and heal off the damage while chipping away at Diancie with Psychic.
252 SpA Diancie Moonblast vs. 252 HP / 4 SpD Cresselia: 144-171 (32.4 - 38.5%) -- 3.1% chance to 3HKO after Leftovers recovery
252 SpA Diancie Moonblast vs. 252 HP / 4 SpD Cresselia: 144-171 (32.4 - 38.5%) -- 98.7% chance to 3HKO after Stealth Rock and Leftovers recovery
4 Atk Diancie Diamond Storm vs. 252 HP / 252+ Def Cresselia: 103-123 (23.1 - 27.7%) -- possible 5HKO after Leftovers recovery
Diancie doesn't have an easy way of killing Cress outside of a Calm Mind set, which is not what we are handling here. Anyways, Cress chips away at Mega Diancie surprisingly quickly with no investment:
0 SpA Cresselia Psychic vs. 0 HP / 0- SpD Diancie: 79-94 (32.7 - 39%) -- 99.8% chance to 3HKO
So after a couple rounds of chipping and Moonlighting, Diancie will either be KOed, or very close to it. Even with some unlucky Moonblast drops, it can still be finished off after two or three Psychics:
-1 0 SpA Cresselia Psychic vs. 0 HP / 0- SpD Diancie: 52-63 (21.5 - 26.1%) -- 3.3% chance to 4HKO
Even with the lowest damage rolls possible, 32.7+21.5+21.5+21.5=97.2, so you can still take it down pretty solidly after a Special Attack drop. Most Cress use Reflect, but I threw Light Screen on for a little extra security. Also, for fun:
0 SpA Cresselia Hidden Power Steel vs. 0 HP / 0- SpD Diancie: 140-168 (58 - 69.7%) -- guaranteed 2HKO