Overview
Boasting an excellent Special Attack and a versatile diverse (contamination: Hydreigon is versatile, that movepool isn't gonna change anytime soon) movepool, Hydreigon is a solid offensive threat in the OU metagame. By virtue of its Dark / Dragon typing, (AC) which gives it numerous resistances, Hydreigon checks common threats such as Rotom-W and Slowbro. With the absence of Aegislash, bulky Psychic-types have become significantly more common, and Hydreigon has an easy time dealing with them. Hydreigon also has a niche over other Dragon-types, particularly Latios and Latias, in that it resists Sucker Punch, allowing it to can easily revenge kill Bisharp due to its Sucker Punch resistance. (I assume the niche is 'being able to rk Bisharp' rather than 'resisting Sucker Punch') In addition, Hydreigon's has a respectable Attack stat with and access to Superpower which targets allow it to target Chansey, an important advantage over many special attackers. While Hyderigon Hydreigon is often outclassed as a wallbreaker, it finds a niche in being as a potent revenge killer when equipped with a Choice Scarf.
However, Hydreigon's average Speed leaves it susceptible to faster attackers such as Latios, Greninja, Keldeo, Terrakion, Mega Lopunny, Mega Gallade, Mega Gardevoir, and Mega Pinsir. While Hydreigon can alleviate this issue by using a Choice Scarf, it then loses out on the ability to switch its attacks and is becomes significantly weaker without a Life Orb. Unfortunately, Hydreigon's STAB moves are resisted by many common Pokemon in OU, (AC) and it is leaving it reliant on hitting them with a coverage move or U-turn. Hydreigon also has many type weaknesses, particularly to Fighting, Dragon, Fairy, and Ice. To make matters worse, the introduction of Fairy-types such as Azumarill, Sylveon, Clefable, and Mega Diancie threaten Hydreigon with a 4x super effective STAB move while resisting or being immune to Hydreigon's STAB moves.
Life Orb Attacker
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name: Life Orb Attacker
move 1: Draco Meteor
move 2: Dark Pulse
move 3: Flash Cannon / Fire Blast
move 4: Superpower / Earth Power
item: Life Orb
evs: 4 Atk / 252 SpA / 252 Spe
nature: Mild / Modest
Moves
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Draco Meteor is Hydreigon's strongest STAB move that and deals massive damage to any Pokemon that is not a Fairy- or Steel-type or Chansey. Dark Pulse hits most Steel-types for neutral damage and provides important coverage on Psychic- and Ghost-types such as Mew, Jirachi, Celebi, and Mega Metagross. Flash Cannon targets Fairy-types such as Mega Altaria, Mega Diancie, Togekiss, and Clefable, (AC) which take little damage from Hydreigon's STAB moves, notably being able to 2HKO or OHKO all of them with some prior damage. Alternatively, Fire Blast hits Steel-types such as Ferrothorn and Klefki, but most other Steel-types are hit hard enough by a Dark Pulse. Superpower deals heavy damage to specially bulky Pokemon such as Heatran, Tyranitar, and Chansey. Earth Power is an alternate coverage move that has a high chance to OHKO Choice Band or Belly Drum Azumarill variants, while also retaining coverage on most Steel-types.
Set Details
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A Mild nature and maximum Special Attack investment allows Hydreigon to hit as hard as possible while retaining the power of Superpower at the cost of Defense. (not worth mentioning because it's just an unwelcome side effect and not a goal of using this nature, some stat has to be lowered and you don't e.g. mention Timid lowering Attack either. The only thing potentially worth mentioning would be why Mild>Rash, which would be something in the vein of 'and preserving Hydreigon's resistances to mostly specially attacking types' if i'm not mistaken) If Superpower is not used, Hydreigon should opt for a Modest nature. However, a Naive (if Mild is used, why not Hasty?) or Timid nature has some merit in being able allowing Hydreigon to outspeed Adamant Mega Charizard X before a Dragon Dance. Maximum Speed investment is important to outspeed as many threats as possible, notably Jolly Mega Pinsir before Mega Evolving and neutral-natured (add hyphen) Kyurem-Black Kyurem-B.
Usage Tips
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Hydreigon functions best as hit-and-run attacker due to Draco Meteor's Special Attack drops. Due to its typing, Hydreigon can also act as an effective pivot into resisted or immune hits attacks it resists or is immune to, ('immune hits' is wrong because the hit isn't immune and it isn't a hit if Hydra doesn't take it, and therefore I also had to rephrase 'resisted hits') from the likes of Rotom-W, Slowbro, and Mega Venusaur. In addition, Levitate gives Hydreigon a helpful immunity to Ground-type attacks from the likes of Hippowdown Hippowdon, Excadrill, Landorus-T, and Gliscor. However, most of these Pokemon have an alternate STAB or coverage move to hit Hydreigon with, so it must be careful when attempting to switch in. In most scenarios, firing off a Draco Meteor or Dark Pulse is not optimal due to the prevalence of Fairy- and Steel-types, so it is important to be smart with predicting well and using use the appropriate coverage move for their targets on the switch.
Team Options
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Steel-types such as Heatran, Magnezone, Mega Scizor, Mega Metagross, and Ferrothorn are great partners for Hydreigon, (AC) as they resist Fairy-, Dragon-, and Ice-type attacks. Heatran and Ferrothon provide Stealth Rock support to help Hydreigon achieve some impoatant important 2HKOs or OHKOs, (such as?) but they compound a Fighting weakness. Fairy-types such as Clefable and Azumarill are good partners to switch into opposing Dragon-types. Cleaners such as Talonflame and Mega Metagross appreciate Hydreigon's wallbreaking ability and have decent synergy with Hydreigon.
Revenge Killer
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name: Revenge Killer
move 1: Draco Meteor
move 2: Dark Pulse
move 3: Earth Power
move 4: U-turn
item: Choice Scarf
evs: 4 Atk / 252 SpA / 252 Spe (why Atk EVs if there's no physical move?)
nature: Modest / Timid
Moves
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Draco Meteor is Hydreigon's strongest STAB move and OHKOs OHKOes many offensive Pokemon after some prior damage. Dark Pulse is a strong secondary STAB move to hit Psychic- and Ghost-types such as Jirachi, Mew, Celebi, and Mega Metagross. Earth Power provides important coverage on Steel- and Fairy-types such as Mega Diancie, Klefki, Tyranitar, Bisharp, Heatran, and Empoleon. U-turn keeps up offensive momentum and gets off important chip damage on the switches that Hydreigon often forces due to its offensive typing and power. Fire Blast can be used to hit Scizor and Ferrothorn much harder than Earth Power, but it leaves Hydreigon unable to significantly damage Mega Diancie and Heatran. Focus Blast has a merit of being is able to OHKO Mega Gyarados after Stealth Rock damage, (RC) but does not have much use outside of that.
Set Details
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Maximum Special Attack and Speed investment allows Hydreigon to outspeed as many offensive Pokemon as possible. A Modest nature is preferred to OHKO Mega Lopunny, Mega Metagross, and Mega Diancie with the appropriate coverage move after Stealth Rock damage. However, a Timid nature outspeeds allows Hydreigon to outspeed other Choice Scarf users such as Landorus-T, but the decrease in power is unwelcome.
Usage Tips
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Equipped with a Choice Scarf, Hydreigon can revenge kill many faster offensive Pokemon such as Mega Sceptile, Greninja, and Latios. U-turn is generally the safest attack to go for early on, as it keeps up offensive momentum and gets off important chip damage. Hydreigon should not use Draco Meteor early on, as there are plenty of Pokemon that resist it and bulky setup sweepers can take advantage of the Special Attack drops. Hydreigon should use the appropriate coverage move when revenge killing a foe, such as targeting a Mega Metagross with Dark Pulse or hitting a frailer Pokemon such as Greninja with a strong Draco Meteor.
Team Options
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Volt Switch users such as Rotom-W, Raikou, and Magnezone are great partners to form a volt-turn VoltTurn core with Hydreigon, which is important for offensive pressure. In addition, Magnezone traps the Steel-types not hit by Hydreigon's chosen coverage move, (not too sure here so not changing yet but if you mean Earth Power you should just say so. Just mentioning because all these mons are Fire Blast targets and Magnezone's not gonna like trapping an Earth Power target such as Heatran) such as Ferrothorn, Skarmory, and Scizor. Other bulky Steel-types such as Heatran, Ferrothorn, and Scizor are good teammates to switch into Fairy-, Bug-, Dragon-, and Ice-type attacks. The former two provide Stealth Rock support, (AC) which helps Hydreigon achieve some important OHKOs or 2HKOs, (any examples?) but they compound a Fighting weakness. Fairy-types such as Azumarill and Clefable are also good partners to sponge hits from opposing Dragon-types. Nasty Plot + Baton Pass Celebi is a great partner that can pass Special Attack boosts to Hydreigon, (RC) and has good synergy with it, (AC) as it can check some Fighting- and Fairy-types such as Azumarill, Terrakion, and Clefable.
Other Options
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Hydreigon has a variety of other coverage (most moves you mention aren't coverage moves) moves at its disposal to use. Taunt prevents defensive Pokemon such as Ferrothorn and Heatran from using setting up entry hazards or using status moves, but having coverage moves is more important for Hydreigon. Tailwind can be used to somewhat alleviate Hydreigon's below average Speed, while also supporting the team. Iron Tail 2HKOs 2HKOes Sylveon as opposed to Flash Cannon which cannot, but the accuracy is often (either 75% is unreliable or it isn't, there's no situation where it's more reliable than another) very unreliable. Roost is an option for the mixed set, but it is generally not worth sacrificing a coverage move.
Checks & Counters
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**Faster Attackers**: Greninja, Keldeo, Terrakion, and Mega Gardevoir, (RC) can outspeed non-Choice Scarf Hydreigon and OHKO it with their respective STAB moves. The former three can switch in on a Dark Pulse but must be careful of a Draco Meteor.
**Specially Bulky Pokemon**: With the attack drops from Superpower, Hydreigon can't 2HKO Chansey, which can stall him it out. Assault Vest users such as Azumarill and Conkeldurr can stomach Hydreigon's attacks and retaliate with a super effective STAB move.
**Priority**: Sadly, Hydreigon has a weakness to two very strong forms of priority. The most common being is Breloom's Mach Punch, which can OHKO Hydreigon with the assistance of a Life Orb. Same goes for Weavile's and Mamoswine's Ice Shard can do this also. Assault Vest Conkeldurr gets another special mention, because unlike the aforementioned threats it can actually switch in on Hydreigon's attacks.
**Fairy-types**: (add hyphen) Hydreigon's best checks are Fairy-types, (add hyphen and comma) as they are immune to and resist Hydreigon's STAB moves. Azumarill is the most notable one, avoiding as it avoids (construction consistency with second part) the 2HKO from all of Hydreigon's moves with an Assault Vest and can OHKO in return with Play Rough. Other Fairy-types such as Clefable and Mega Diancie can also switch into Hydreigon's STAB moves, but they must be wary of a Life Orb-boosted (add hyphen) Flash Cannon.
