Nominating
Gastrodon as a
counter.
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 248 HP / 8 Def / 252 SpD
Sassy Nature
- Scald
- Earthquake
- Toxic
- Recover
Gastrodon is a very good
counter to Diancie owing to its solid resistance to Diamond Storm, ominously large bulk, access to Recover to continually heal off any attacks thrown its way, and STAB Earthquake and Scald, both of which chunk Diancie for enormous damage, and the latter threatening to burn it, even through Magic Bounce. Moonblast doesn't even 2HKO after Stealth Rock, making Gastrodon a very good
counter to Diancie.
252 SpA Diancie-Mega Moonblast vs. 248 HP / 252+ SpD Gastrodon: 148-175 (34.8 - 41.1%) -- 99.4% chance to 3HKO after Stealth Rock and Leftovers recovery
0 Atk Gastrodon Earthquake vs. 0 HP / 0- Def Diancie-Mega: 188-224 (78 - 92.9%) -- 37.5% chance to OHKO after Stealth Rock
=================================================================================================
Nominating
Ash-Greninja as a
check
Greninja @ Choice Specs
Ability: Battle Bond
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Hydro Pump
- Dark Pulse
- Water Shuriken
- Spikes
While being completely allergic to taking any sort of hit from Diancie whatsoever, and thus being entirely unable to switch in, Greninja outspeeds Diancie and threatens the OHKO with Specs-boosted STAB Hydro Pump, even when untransformed. Even Water Shuriken untransformed kills most of the time if you pull 4 hits or higher, thus making it a decent enough last-resort
check.
252 SpA Choice Specs Greninja Hydro Pump vs. 0 HP / 0 SpD Diancie-Mega: 422-498 (175.1 - 206.6%) -- guaranteed OHKO
252 SpA Choice Specs Greninja Water Shuriken (15 BP) (4 hits) vs. 0 HP / 0 SpD Diancie-Mega: 240-288 (99.5 - 119.5%) -- approx. 81.3% chance to OHKO