Here's a check I've been working on. I haven't tested this thing out extensively, but it checks Mega-D nicely, walls Greninja w/o Extrasensory (which nobody's going to run anyway), and spins to boot!
CTHULHU (Tentacruel) @ Assault Vest
Ability: Liquid Ooze
EVs: 252 HP / 240 Def / 16 SpA
Nature: Relaxed/Bold
- Scald / Hydro Pump
- Sludge Bomb
- Rapid Spin
- Knock Off / Ice Beam
The choice of Scald/HP comes down to whether you value the burn chance more than the OHKO. Of course, Scald puts still a pretty good dent in it:
16 SpA Tentacruel Scald vs. 0 HP / 0- SpD Diancie: 152-180 (63 - 74.6%) -- guaranteed 2HKO
16 SpA Tentacruel Hydro Pump vs. 0 HP / 0- SpD Diancie: 206-246 (85.4 - 102%) -- 87.5% chance to OHKO after Stealth Rock
On the other hand, Tenta can freely switch into any of Diancie's attacks bar Diamond Storm, which is still only a 3HKO before rocks.
4 Atk Diancie Diamond Storm vs. 252 HP / 240+ Def Tentacruel: 147-174 (40.3 - 47.8%) -- 55.5% chance to 2HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 252 HP / 0 SpD Assault Vest Tentacruel: 49-58 (13.4 - 15.9%) -- possible 6HKO after Stealth Rock
252 SpA Diancie Earth Power vs. 252 HP / 0 SpD Assault Vest Tentacruel: 124-148 (34 - 40.6%) -- guaranteed 3HKO after Stealth Rock
I've been interested in Tenta as an anti-meta Pokemon, and with an Assault Vest it has great mixed bulk. There are probably a lot of variations on this set which could work (more SpA investment, maybe Air Balloon for Mega-Beedrill?), but I like this guy as a bulky spinner and Diancie check.
CTHULHU (Tentacruel) @ Assault Vest
Ability: Liquid Ooze
EVs: 252 HP / 240 Def / 16 SpA
Nature: Relaxed/Bold
- Scald / Hydro Pump
- Sludge Bomb
- Rapid Spin
- Knock Off / Ice Beam
The choice of Scald/HP comes down to whether you value the burn chance more than the OHKO. Of course, Scald puts still a pretty good dent in it:
16 SpA Tentacruel Scald vs. 0 HP / 0- SpD Diancie: 152-180 (63 - 74.6%) -- guaranteed 2HKO
16 SpA Tentacruel Hydro Pump vs. 0 HP / 0- SpD Diancie: 206-246 (85.4 - 102%) -- 87.5% chance to OHKO after Stealth Rock
On the other hand, Tenta can freely switch into any of Diancie's attacks bar Diamond Storm, which is still only a 3HKO before rocks.
4 Atk Diancie Diamond Storm vs. 252 HP / 240+ Def Tentacruel: 147-174 (40.3 - 47.8%) -- 55.5% chance to 2HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 252 HP / 0 SpD Assault Vest Tentacruel: 49-58 (13.4 - 15.9%) -- possible 6HKO after Stealth Rock
252 SpA Diancie Earth Power vs. 252 HP / 0 SpD Assault Vest Tentacruel: 124-148 (34 - 40.6%) -- guaranteed 3HKO after Stealth Rock
I've been interested in Tenta as an anti-meta Pokemon, and with an Assault Vest it has great mixed bulk. There are probably a lot of variations on this set which could work (more SpA investment, maybe Air Balloon for Mega-Beedrill?), but I like this guy as a bulky spinner and Diancie check.