Shadow Hydreigon

Hydreigon has had a bit of a rough time in Pokemon Go; it dropped in two metas; one underwhelming, and one where it stood no real chance of comparative viability. The introduction of Brutal Swing finally broke Dark out of its cage, but then we saw powerful Dark Type Shadows and Mega Evolutions get the move as well, pushing Hydreigon down to relative obscurity.

Well, guess what? It's time for the Brutal Pokemon to take its rightful place as one of the top Dark Types in the game. Shadow Hydreigon breaks out of mediocrity and becomes a legitimate threat as a Dark Type attacker, supported by decent bulk and a great defensive sub-typing... so long as whatever it's fighting doesn't have Fairy Type coverage moves. However, it will likely never become an optimal pick among Dragon Types.

Best PvE Offensive Moveset

• You're going to want to run Bite and Brutal Swing* for Hydreigon, as Dark is where it genuinely excels.
• Dragon Breath and Dragon Pulse can work for Dragon damage, but it's genuinely unimpressive.
• Flash Cannnon and Dark Pulse fall behind.

Best PvE Defensive Moveset

• Dragon Pulse and Bite are basically clones of one another, but the former pulls ahead due to the fact that it is off-type from Brutal Swing*, which is the Charged Move of choice.
• Flash Cannon has counter-coverage against Fairy and Ice Types, but Hydreigon likely won't survive long enough to launch it.
• Dragon Pulse and Dark Pulse are inferior.

Dragon Breath and Bite both hit hard with STAB, though the former has significantly better energy gains which makes it the superior choice. 

Brutal Swing* is a move limited to Community Day events and Elite TMs, but its lesser energy cost improves Shadow Hydreigon's performance and consistency in terms of damage output and shield pressure. Dark Pulse costs noticeably more energy, and it is generally recommended to postpone evolving Zweilous until future Community Day recap events in order to obtain Brutal Swing instead. Dragon Pulse isn't as necessary mainly due to requiring more energy, though it can threaten OHKOs against opposing Dragon-types. Flash Cannon is Shadow Hydreigon's only hope at threatening Fairy-type targets in some way, but its expensive cost makes it extremely unreliable.