I would like the utility check set to be replaced with this:
Kyurem-Black @ Leftovers
Ability: Teravolt
EVs: 60 Spd / 252 Atk / 196 SDef
Adamant Nature
- Fusion Bolt
- Dragon Claw
- Ice Beam
- Roost
I don't see the point of Dragon Tail, so many Speed EVs, and max HP investment. You mention that Dragon Tail helps against Pokemon such as Manaphy and DD Mega Gyarados, but max Atk+ Fusion Bolt is a way better solution for those, OHKOing both after SR damage. Dragon Claw is your strongest move, and is great against Pokemon such as Rotom-W, Greninja, Mega Charizard X, opposing Kyu-B (you counter Sub variants), (Mega) Tyranitar, Chansey, Conkeldurr, Gengar, Mamoswine, and Mega Manectric, all important targets. Dragon Tail is both pointless with so much Speed investment and not needed as Kyu-B can beat the things it checks with sheer power anyway.
So much Speed is not needed, as Excadrill beats you anyway with Iron Head if it's offensive and is outsped anyway if it's defensive. What other Pokemon do you want to outspeed really? There is Dragonite and Mega Gyarados, true, which means that going with 263 Speed is a fine option too (outspeeds Jolly Bisharp too), but i prefer the extra bulk, though i really don't mind, both are really good spreads and better than the one you have. Other than that, 60 Speed gives you everything you really need, by outspeeding and hitting for good damage a very relevant threat, Adamant Bisharp, as well as +Speed Aegislash.
As for SpD > HP investment, it's simple really. The majority of the Pokemon that Kyurem-B checks are specially based, and you get way more special bulk by investing in SpD. Here are some calcs:
- +3 252 SpA Life Orb Manaphy Ice Beam vs. 0 HP / 196 SpD Kyurem-B: 237-279 (60.6 - 71.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
- +3 252 SpA Life Orb Manaphy Ice Beam vs. 196 HP / 0 SpD Kyurem-B: 290-342 (65.9 - 77.7%) -- 25% chance to OHKO after Stealth Rock
- 200+ SpA Teravolt Kyurem-B Ice Beam vs. 0 HP / 196 SpD Kyurem-B: 132-156 (33.7 - 39.8%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
- 200+ SpA Teravolt Kyurem-B Ice Beam vs. 196 HP / 0 SpD Kyurem-B: 160-190 (36.3 - 43.1%) -- 32% chance to 2HKO after Stealth Rock and Leftovers recovery
- 252 SpA Life Orb Protean Greninja Ice Beam vs. 0 HP / 196 SpD Kyurem-B: 146-173 (37.3 - 44.2%) -- 52.3% chance to 2HKO after Stealth Rock and Leftovers recovery
- 252 SpA Life Orb Protean Greninja Ice Beam vs. 196 HP / 0 SpD Kyurem-B: 177-211 (40.2 - 47.9%) -- 98.8% chance to 2HKO after Stealth Rock and Leftovers recovery
If outspeeding Adamant Bisharp is not important, as you can't OHKO it and +2 Sucker Punch OHKOes after SR, you can go with max Atk+ / max SpD, so that you almost always survive two Ice Beams from Greninja after SR, survive way more often a Timid Fire Blast from Char Y after SR, and prolly a few more nifty things.
Finally, as for the nature, Adamant > Hasty, because you want to preserve your Defense to check Mega Gyarados better:
- 252+ Atk Mold Breaker Mega Gyarados Earthquake vs. 0 HP / 0 Def Kyurem-B: 136-161 (34.7 - 41.1%) -- 2.3% chance to 2HKO after Stealth Rock and Leftovers recovery
- 252+ Atk Mold Breaker Mega Gyarados Earthquake vs. 0 HP / 0- Def Kyurem-B: 152-179 (38.8 - 45.7%) -- 84.4% chance to 2HKO after Stealth Rock and Leftovers recovery
Taking less damage from priority users such as Talonflame, Bisharp, and Mega Mawile on 1 on 1 scenarios is great as well. Ice Beam is good enough even with a -SpA nature, and it doesn't miss any important OHKO/2HKO as far as i am concerned.
tll;dr the new spread gives way more power and makes Kyurem-B not easy at all to wall without dedicated defensive cores, and still checks the Pokemon it needs to, in fact it checks them even better than the old spread.
Here are the possible spreads and their advantages:
60 Spd / 252 Atk / 196 SDef with Adamant: Outspeeds Adamant Bisharp and +Speed Aegislash while still checking most of the stuff it is supposed to well
148 Spd / 252 Atk / 108 SDef with Adamant: Outspeeds Mega Gyarados and Dragonite, but has a bigger chance to get 2HKOed by Greninja and loses some special bulk in general.
252 Atk / 4 SpA / 252 SDef with Adamant: Almost always avoids the 2HKO from Greninja's Ice Beam, and handles some special attackers better