Moves
Protect allows Yanmega to scout for Z-Moves and guarantees that it gets a Speed boost. Air Slash is Yanmega's main Flying-type STAB move, capable of dealing massive damage to Pokemon like Kommo-o and Mega Heracross. Giga Drain allows Yanmega to bypass Pokemon that otherwise check it like Mega Diancie, Seismitoad, Mega Swampert, and Gastrodon. Bug Buzz can be used instead of Giga Drain, since it is Yanmega's strongest STAB move and allows it to bypass Dark- and Psychic-type Pokemon like Hydreigon and Mew, but Bug-type teams don't have much trouble dealing with the teams these Pokemon are on, thus making it kind of unnecessary. Ancient Power can be used to hit Volcarona, Mega Pinsir, and Mega Charizard Y for extra damage, all of which are threats to Bug-type teams. The reason to use Ancient Power instead of Hidden Power Rock is because of its chance to boost all Yanmega's stats, which can be quite useful at times. Hidden Power Ice allows Yanmega to beat Pokemon 4x weak to it, mainly Dragonite, Garchomp, and Landorus-T. Hidden Power Fire is yet another option to hit Scizor, Mega Scizor, and Ferrothorn, which are all capable of taking Yanmega's STAB attacks.
Set Details
Maximum Speed investment with maximum Special Attack investment and a Modest nature maximizes Yanmega's offensive potential. A Timid nature can be used to outspeed certain Choice Scarf users at +1 like Heracross and Landorus-T, which it'd otherwise get outsped by. Speed Boost allows Yanmega to gain a boost in Speed every turn, allowing it to outspeed a majority of the metagame after just one turn. Life Orb further enhances Yanmega's damage output.
Usage Tips
Yanmega is best used as a late-game cleaner to cleave through the opposing team once it is weakened. Be mindful when using Protect, since other setup sweepers like Volcarona might benefit from this turn to set up for themselves. It's best to not switch Yanmega in on Stealth Rock so Yanmega can switch in more times during the match, and it's best to keep Yanmega as healthy as possible in general, especially if the opposing team has a priority user. Yanmega's bulk is pretty bad, so it should avoid directly switching in on an attack in general; it's best to bring Yanmega in via a fainted teammate, on a double switch, or via a slower U-turn from teammates like Mega Scizor. Yanmega can also be brought in on Pokemon like Mandibuzz, Jellicent, Gastrodon, and Quagsire, which tend to have a passive nature. Try to avoid getting paralyzed, because this makes Yanmega rely on Speed boosts too much. Use Yanmega's coverage moves to hit what is on the opposing side of the field, as it is generally ineffective to predict with Yanmega. It should also be considered if it is actually worth it to reveal Yanmega's coverage moves early-game, since the opponent might scout for it, and knowing Yanmega's coverage moves allows them to play around it accordingly.
Team Options
When using Yanmega, entry hazard removal Pokemon like Armaldo and Forretress are required because of its 4x weakness to Stealth Rock. Armaldo and Forretress are both also capable of setting up hazards, allowing Yanmega to get KOs more easily. Assault Vest Araquanid is a good teammate because it can take Ice- and Fire-type attacks with relative ease for Yanmega thanks to its ability Water Bubble. Galvantula is capable of setting up Sticky Web for Yanmega, which allows it to outspeed more Pokemon and be less reliant on Speed Boost, as well as threatening Flying-types with its STAB Electric-type attacks. Pokemon like Choice Scarf Buzzwole and Choice Scarf Heracross can be useful if Yanmega is lacking Bug Buzz, as they are capable of dealing with Psychic-, Dark-, and Rock-types, which Yanmega tends to struggle against. Yanmega also appreciates Pokemon that are able to dispose of Steel-types, since it has a hard time dealing with them; Volcarona, Buzzwole, Heracross, Mega Heracross, and Mega Pinsir are capable of doing so. Pokemon that can weaken walls like Mega Pinsir, Volcarona, Mega Heracross, and Mega Scizor are very useful for Yanmega, as they ease its job to clean up.