Diancie

Fairy

Mega Diancie's amazing Attack, Special Attack, and STAB options in Moonblast and Diamond Storm make it the best mixed wallbreaker for Fairy teams. After Mega Evolving, Mega Diancie's Speed is incredible for a wallbreaker, and it is able to check fast attackers like Mega Charizard Y and Nihilego that are otherwise difficult for Fairy to deal with. Its huge movepool makes it unpredictable, and it has many coverage options that it can use. In fact, with Psychic, Mega Diancie is the best check to Poison teams and is the only reliable way to break their defensive core. Access to Stealth Rock allows Mega Diancie to support its team further, while setup moves like Rock Polish make Mega Diancie capable of sweeping on its own, expanding its possibilities. Unfortunately, Mega Diancie's bulk is rather terrible, and its Rock typing makes it weak to many priority users such as Azumarill and Mega Scizor. While base 110 Speed is very good against defensive teams, Mega Diancie is still easily outsped by offense teams if it doesn't have Rock Polish. When combined with its poor bulk, Mega Diancie is very easy to revenge kill. Lastly, Mega Diancie suffers from serious four-moveslot syndrome, as dropping a coverage move for utility leaves Mega Diancie walled by key threats, meaning its vast movepool cannot always be fully utilized.

Rock

Mega Diancie stands out as the best Mega Evolution and mixed attacker for Rock teams thanks to its fantastic Attack and Special Attack stats and great movepool. These traits are accompanied by an impressive 110 Speed stat, allowing Mega Diancie to outspeed threats such as Keldeo and Terrakion. Mega Diancie takes great advantage of its Attack stat with Diamond Storm, a powerful and reliable Rock-type STAB move with a 50% chance to increase its Defense by 2 stages. This can be very helpful against threatening physical attackers like Breloom and defensive Mega Scizor, since it survives an unboosted Bullet Punch when at +2 Defense. With Magic Bounce, Mega Diancie provides fantastic utility, bouncing back entry hazards such as Galvantula's Sticky Web as well as status moves like Thunder Wave and Will-O-Wisp. Its part-Fairy typing along with a strong Fairy-type STAB move in Moonblast allows it to beat threats to Rock teams such as Mega Sableye. Its great movepool also allows it to provide a plethora of helpful coverage. Unfortunately, Mega Diancie's terrible bulk and defensive typing make it largely vulnerable to common priority moves such as Aqua Jet and Bullet Punch. Even with Mega Diancie's fantastic Speed, it's still susceptible to revenge killers like Excadrill in sand, Tapu Koko, and Choice Scarf Jirachi. Mega Diancie also has difficulty fitting all of its beneficial coverage options onto a single set.