Hurting Hydreigon
Hydreigon is a beast, as expected of the Pseudo-Legendary of the 5th generation, but the introduction of fairies was one of the most damaging events to Hydreigon, as a new 4x weakness on top of its normal limitations causes Hydreigon to struggle. It did not help that the metagames after it was introduced favored fairy types, and the current Sun and Moon metagame infested with the Tapus. Furthermore, weakness to common offensive types like Fighting, Ice and Dragon, continue to make Hydreigon's life difficult, with many teams able to maim or OHKO it. A base 98 speed falls short of the base 100 mark that many potent threats are at, like Salamence, Mega Charizard X (and Y with Dragon Pulse/Air Slash), Flygon, Staraptor, Mega Gardevoir, Manaphy, Volcarona, Mega Medicham, etc. and this is not even counting the numerous >100 base speed threats that have been introduced since its introduction. Gengar with Dazzling Gleam, Lati@s, Mienshao, Cobalion, Terrakion, etc. Can all come in on a free switch and threaten nonScarf variants of Hydreigon out, or go for the OHKO with with the appropriate move.
Fairies, as mentioned before, give Hydreigon a horrible time, have a 4x effective STAB and resisting both of Hydreigon's stabs, causing many to run Flash Cannon in an attempt to deter incoming fairies, but due to their high SDEF, many fairies are still a serious problem. Alolan Ninetails is in the same vein as those above, hating Fire Blast and Flash Cannon, but otherwise able to OHKO with a STAB Moonblast or Blizzard. Mega Gardevoir, and Florges can easily come in on a special set, tank anything twice and OHKO back with a STAB fairy attack.
Hydreigon's typing is also unfortunate in that it leaves it vulnerable to priority moves like Mach Punch, First Impression and Ice Shard. Admittedly, Golisopod needs a free switch in, or it will be forced out by Emergency Exit, but First Impression is a near guaranteed OHKO with max investment. Breloom, Conkeldurr, and Mamoswine all fail to OHKO Hydreigon with their Priority moves, but all will 2HKO with them, and thus are great a picking off a weakened Hydreigon. Assault Vest Conkeldurr is especially problematic, as it can eat a Specs Draco Meteor (60.6%-71.7%) and can OHKO with Drain Punch (that is still not a free invitation to switch in). Due to Hydreigon's coverage, the others cannot switch in with impunity, lest they get roasted. Tapu Lele is a good shield against priority moves and can handle all but Mamoswine easily. Doublade also walls all of these (though must be wary of Conkeldurr's Knock Off).
252 Atk Golisopod First Impression vs. 0 HP / 0 Def Hydreigon: 314-372 (96.6 - 114.4%) -- 81.3% chance to OHKO
252+ Atk Conkeldurr Mach Punch vs. 0 HP / 0 Def Hydreigon: 168-198 (51.6 - 60.9%) -- guaranteed 2HKO
252+ Atk Technician Breloom Mach Punch vs. 0 HP / 0 Def Hydreigon: 236-278 (72.6 - 85.5%) -- guaranteed 2HKO
240 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 0 Def Hydreigon: 187-221 (57.5 - 68%) -- guaranteed 2HKO
Chansey and Blissey are two of the biggest thorns in Hydreigon's side, and will win against most special sets (read 90+% of Hydreigon), with only a mixed set running Superpower as well as Modest Specs Focus Blast eeking out a 3HKO, so having a partner to take care of them (Conkeldurr, Buzzwole, etc.) is recommended.
That being said, Hydreigon is a great pivot, boasting 6 resistances and 2 outright immunities give it numerous opportunities to switch in, and thanks to its 92/90/90 bulk, it can take hits fairly well. Furthermore, Levitate allows Hydreigon to ignore most entry hazards, while taking only chip damage from Stealth Rock, meaning it can abuse Choice sets more effectively than most pokémon. Hydreigon's ATK, while unimpressive at base 105, is definitely viable, enabling Hydreigon to run a multitude of sets, giving it a degree of unpredictability. Hydreigon greatly appreciates Sticky Web support as it enables to outspeed threats that would otherwise have it on the ropes. Poison types like Amoongus, Alolan Muk, Tentacruel, Mega Venusaur, Gengar (a little frail)... pivot well with Hydreigon, resisting Fairy, Fighting and Bug attacks, while enjoying the immunities to Psychic and Ground Hydreigon has to offer. Hydreigon makes a great VoltTurn partner, immune to the Ground moves that most Volt Switch pokémon (not packing Levitate...) hate.
