Moves
Moonblast is a Fairy-type STAB attack that allows Mega Diancie to hit a significant amount of the tier for neutral damage. It's also useful for hitting Marshadow, Mega Salamence, Zygarde-C, Rayquaza, Mega Sableye, and Tyranitar.
Diamond Storm is a useful 100-Base Power STAB move that hits Flying-type Pokemon such as Ho-Oh for super effective damage. It also takes advantage of Mega Diancie's high Attack stat and allows it to easily take care of Pokemon with high Special Defense such as Blissey, Primal Kyogre, and Gothitelle. Another useful perk of Diamond Storm is the fact that it has a 50% chance to boost Mega Diancie's Defense by two stages, allowing it to survive crucial attacks such as a Precipice Blades from an uninvested Primal Groudon.
Earth Power is useful for dealing with common Fairy-resistant Pokemon such as Primal Groudon, Mega Gengar, Magearna, and Toxapex. It should be noted, though, that there are a large number of Steel-type Pokemon that are unfazed by Earth Power, either due to a secondary typing that lets them take neutral damage from Earth Power or an ability combined with a lot of natural bulk that allows them to take minimal damage from Earth Power.
Toxic punishes just about every non-Steel- and Poison-type Pokemon that switches in, wearing them down and making them easier for Diancie and other teammates to KO.
Running Stealth Rock on Mega Diancie means that teammates such as Necrozma-DM and Primal Groudon don't have to run it and are then capable of running more offensive sets.
Heal Bell allows Mega Diancie to act as an offensive cleric and can be very useful against offensive teams, where Mega Diancie can struggle to put in work and therefore help its teammates by curing them of status and slightly wearing down foes.
Endeavor allows Mega Diancie to take advantage of its low HP stat and make offensive threats easier to wear down.
Set Details
Maximum EVs in Speed let Mega Diancie take advantage of its base 110 Speed tier.
24 EVs in Attack guarantee a 2HKO on Blissey, meaning that it's unable to PP stall Mega Diancie with Soft-Boiled.
The remainder of the EVs are placed in Special Attack for maximum damage output.
A Naive nature is chosen so Mega Diancie is able to take advantage of its Speed stat and potential Defense boosts from Diamond Storm as well as effectively act as a mixed attacker.
Diancite is required for Diancie to become Mega Diancie.
Usage Tips
Try to Mega Evolve Diancie as soon as possible, as that way you're able to utilize its ability more effectively. Note that Clear Body can occasionally be useful when Sticky Web is on the field or to prevent Intimidate Attack drops from Salamence before it Mega Evolves, but in reality, this is still hard to justify not Mega Evolving, since Mega Diancie has a solid matchup against Sticky Web setters and is able to KO Mega Salamence with Moonblast anyway.
While you should avoid letting Mega Diancie take any damage at all, it is best to bring Mega Diancie in against status or utility moves or against attacks that it resists such as Dark- and Flying-type moves.
Although Diancie can quite easily gain a +2 Defense boost with Diamond Storm, it is still better to avoid letting it take supereffective physical attacks, as it is rarely able to survive these, even at +2.
Mega Diancie is a very matchup-dependent Pokemon, as it can be a potential wincon against stall teams after defensive threats such as support Arceus-Ground and Arceus-Water have been removed from the field; against offensive teams, however, just use Mega Diancie to try to reflect entry hazards and other status moves, wear down foes with attacks, and potentially heal the status conditions of teammates with Heal Bell.
Although Diamond Storm provides great benefits, be wary that it only has 8 PP, so avoid using it too liberally. It is especially important to conserve the PP of Diamond Storm when facing stall teams, as Mega Diancie needs Diamond Storm to KO threats such as Blissey and Ho-Oh.
Team Options
Gothitelle makes a brilliant partner for Mega Diancie, as it can trap the support Arceus formes that threaten Mega Diancie as well as other defensive Pokemon that pose a great threat to Mega Diancie such as Celesteela and Ferrothorn. It is also worth noting that the core of Gothitelle + Mega Diancie beats every relevant Defogger in the tier one-on-one, which makes it easier for Mega Diancie to set up Stealth Rock and keep it on the field for the majority of the game.
Necrozma-DM is a brilliant partner for Mega Diancie, as it is able to deal with Xerneas, Arceus-Water, and Arceus-Fairy lacking Will-O-Wisp. In return, Necrozma-DM appreciates Mega Diancie's ability to deal with Ho-Oh and Yveltal, and Mega Diancie's Stealth Rock support lets Necrozma-DM run more offensive sets like Double Dance. Support Necrozma-DM also makes a useful partner for Mega Diancie, as it is a strong Steel-type that shuts down some support Arceus formes such as Arceus-Ground and Arceus-Fairy.
Ho-Oh makes a nice partner, as it can threaten the Steel-types that threaten Mega Diancie, and it also appreciates Mega Diancie's ability to prevent entry hazards from being set.
Arceus-Ground is a useful partner, as it provides the team with a secondary means of keeping hazards off the field with Defog, as well as threatening many of the Ground- and Steel-type Pokemon that beat Diancie, such as Primal Groudon, Excadrill, Mega Lucario, and Zygarde-C.
Giratina-O can take advantage of the status that Mega Diancie can potentially reflect with Magic Bounce and appreciates Diancie's resistance to Dark-type attacks. Furthermore, it can help support Mega Diancie by Defogging away hazards.
Primal Groudon is also a nice partner for Mega Diancie, as it is able to check Necrozma-DM more effectively than Arceus-Ground and is also an effective check to Xerneas.
Both offensive and defensive variants of Zygarde-C make good partners for Mega Diancie, as they provide the team with a check to Primal Groudon, Necrozma-DM, Earthquake Mega Salamence, offensive Arceus-Ground sets, Mega Lucario, and Ho-Oh. Offensive variants of Zygarde-C particularly appreciate Diancie's ability to deal with Yveltal.
Marshadow makes a nice offensive partner for Mega Diancie, as it provides teams with a means of threatening Necrozma-DM, Ultra Necrozma, and common Ghost-types such as Mega Gengar, as well as providing teams with priority, making it easier to clean off weakened foes thanks to Shadow Sneak. Marshadow also appreciates Mega Diancie dealing with common threats to it such as Yveltal and checking Mega Salamence and Ho-Oh, which are two Pokemon that Marshadow can struggle with depending on whether it runs Hidden Power Ice or Rock Tomb.
Arceus-Water makes a good defensive partner for Mega Diancie, as it is able to check a number of Pokemon that Mega Diancie struggles to deal with, such as Arceus-Ground, Primal Groudon, Ho-Oh, Mega Salamence, and Primal Kyogre sets lacking Thunder. Arceus-Water sets running Will-O-Wisp also have a decent matchup against Necrozma-DM and Mega Lucario, as it resists their Steel-type attacks and can burn them.
Yveltal is a good offensive partner for Mega Diancie, since it threatens Necrozma-DM, Marshadow, and Mega Gengar. It also provides the team with a Ground immunity, which is crucial to have in a metagame dominated by Primal Groudon.