Yanmega

[WouW]

#469 Yanmega
Bug/Flying
Speed Boost: At the end of a turn, raises Speed by one stage
Tinted Lens: If the opponent resists your attack, its power is doubled
86HP/76Atk/86Def/116SpA/56SpD/95Spe

Choice Specs

Yanmega @ Choice Specs
Modest/Tinted Lens
252SpA/252Spe/4Def
Bug Buzz
Air Slash
Hidden Power Ice/Hidden Power Ground
Hypnosis/U-Turn

Thanks to Tinted Lens, your moves will hit very hard unless your opponent brings in a special wall or something that 4x resists your moves. Bug Buzz will be your primary attack and Air Slash is there for secondary STAB. Your third move will be either Hidden Power Ice to deal with the dragons that are 4x weak to Ice, and Hidden Power Ground will OHKO Magnezone and Heatran who will otherwise wall you. The fourth move is largely filler but you can use Hypnosis to shut down one of Yanmega's counters, or U-Turn to scout.

If you want more speed at the cost of power, you could either replace Tinted Lens with Speed Boost, replace your Specs with a Scarf or change the nature from Modest to Timid (making you outspeed neutral base 105 and positive base 90).

SubPetaya

Yanmega @ Petaya Berry
Modest/Speed Boost
252SpA/252Spe/4Def
Substitute
Bug Buzz
Air Slash
Hidden Power Ice/Hidden Power Ground

Make sure you set your HP IV to 30. That way, Petaya will activate after only one Sub+Stealth Rock, or three Substitutes. After getting one or more speed boosts and Petaya Berry is activated, you're ready to sweep with 546 Special Attack and 433 (after 1 Speed Boost) or 578 (after 2 Speed Boosts) or even more speed. Watch out for Sandstorm, as it can potentially ruin your sweep.

Substitute #2

Yanmega @ Liechi Berry
Rash/Speed Boost
252Atk/252SpA/4Spe
Reversal
Substitute
Bug Buzz
Air Slash

A rather gimmicky set, but it could work. Keep your HP EV at 31 this time, as this will allow you to Sub 2 times with Stealth Rock or 4 times without - this will bring your HP to 1 (beware of Sandstorm) and give Reversal 200 Base Power, OHKOing 252Def/0HP Calm Blissey who would otherwise wall all your special attackers. Petaya Berry can be used over Liechi to hide the fact that you use Reversal, but you will need a Naughty nature to OHKO Blissey, and even then a physically bulkier Blissey might be able to survive it. Reversal can also hit other special walls, such as Snorlax or Regice.

Other options:

Protect can be used for a free Speed Boost, but it may be hard to find a slot for it. Shadow Ball can be used to deal with Rotom-A. Yanmega doesn't have a lot of other options.

EVs:

Max Special Attack and Speed, and the remaining point should go to Defense; this keeps your HP uneven and allows you to switch into Stealth Rock twice. The Substitute #2 set should have max Attack to be able to take on Blissey, and will primarily get its speed from Speed Boost.

Counters:

Blissey, Snorlax and Regice are famous for their special walling abilities, and the latter two often carry SleepTalk sets so even after Hypnosis you're still not done with them. Heatran and Magnezone resist all but are destroyed by Hidden Power Ground. Rotom-A with Special Defense investment can force Yanmega out easily, unless Yanmega has Shadow Ball. Sandstorm by itself can screw up Substitute sets. Other important things are that Yanmega loses 50% of its health from Stealth Rock, and has terrible Special Defense and therefore isn't that hard to beat.