Diancie

Rock

Diancie is the best Trick Room setter on Rock, as well as a decent Stealth Rock setter, and it performs well as both a bulky attacker and Trick Room sweeper thanks to its decent attacking stats and magnificent 50 / 150 / 150 bulk; on the other hand, Mega Diancie is a great mixed attacker with the ability to OHKO many threatening Pokemon in the Monotype metagame thanks to its spectacular attacking stats and Speed, as well as decent 50 / 110 / 110 bulk. On Rock teams, Mega Diancie is one of the best wallbreakers due to its mixed STAB attacks, Diamond Storm and Moonblast. Its Fairy typing also makes it a great offensive check to most Fighting-types, and it outspeeds most of them too; it puts offensive pressure on Dragon teams too because it prevents them from using their Dragon-type attacks against Rock teams reliably. Mega Diancie is also a decent stallbreaker as well due to Magic Bounce, as it can reflect status moves and damage most status users with its attacks. However, while a Rock / Fairy typing is great for Rock teams, it makes Diancie doubly weak to Steel-type moves, of which Bullet Punch is especially threatening. Diancie's awful base 50 Speed also means that it almost always needs to use Protect upon Mega Evolving to guarantee that it can survive until the next turn to begin sweeping.

Fairy

Upon Mega Evolving, Diancie gains amazing Attack and Special Attack combined with a great Speed stat, making it a dangerous mixed attacking threat to many teams. However, it loses some of its bulk. Its Fairy / Rock typing gives it key resistances to Fire and Flying, and allows it to hit every type except Steel for at least neutral damage, but it does give it weaknesses to Water, Ground, Grass, and Steel. It can, however, run a move to hit Steel-types with in Hidden Power Fire or Earth Power. Mega Diancie has access to a very reliable Fairy-type STAB attack in Moonblast, as well as a Rock-type STAB move in Diamond Storm, which allows it to check Fire- and Flying-types. It also has access to setup moves such as Calm Mind and Rock Polish, but running one of those means it has to sacrifice Protect. Despite its great Speed, Mega Diancie has a weakness to some priority moves such as Bullet Punch and Aqua Jet. Its Magic Bounce ability allows it to reflect status moves such as Thunder Wave and Toxic, as well as entry hazards. Before Mega Evolving, Diancie's Speed is very low, so it has to rely on Protect to get the boost in Speed. Although Mega Diancie has decent Defense and Special Defense, its HP is very low, which means it can't take non-resisted hits that well.