Facing a Meloetta is like biting into a Bertie Bott's Every Flavour Bean—you will never know what exactly it could do. Indeed, out of the new additions to grace Pokemon, Meloetta is arguably the most interesting of them all, albeit the most overlooked. What makes her so unique is the ability to change between two formes that have almost opposite counters, and in the hands of a skilled player, this can cause huge problems for their opponents. One thing that does frustrate a lot of players is that fact that Meloetta cannot start in Pirouette forme immediately, which is arguably her more threatening forme. However, her Aria forme is far from useless, and can easily function as either an offensive or defensive Pokemon, depending on whatever you need. Her lack of physical bulk and relatively low Speed might make the other Psychic-types of OU more attractive, so always remember to play to Meloetta's strengths. In short, we might not be able to grasp the full extent of Meloetta's abilities for some time to come, but there is one thing we do know, and that is that she can be devastating in the right hands.
The purpose of this set is to fully exploit Meloetta's forme-changing abilities. The idea is to, with prediction, stay a step ahead of your opponent by continuously switching between Aria and Pirouette formes. It is especially convenient when a forme's counter is weak to the other forme, which means if Meloetta stays in the advantageous forme, she will retain offensive momentum in a similar way to VoltTurn cores. For example, Skarmory switches out of Aria forme as it fears Thunder, but as your opponent switches in Blissey, Meloetta uses Relic Song to transform into her Pirouette forme, and Blissey has to switch out again in fear of Close Combat. With entry hazard support, this can force a lot of switches and cause a lot of damage very quickly.
Relic Song is obviously the integral part of this set, as it facilitates the forme-changing that is unique to Meloetta. It is a decently powerful STAB move backed by base 128 Special Attack, while having a 20% chance to put an opponent to sleep, which is always helpful. Close Combat is Meloetta's main STAB attack while in Pirouette forme. Thunder is preferred in the third slot since firstly, rain is extremely common, while secondly, its power can help Meloetta get some crucial KOs, such as on Tornadus-T while in Pirouette forme. It also lands 2HKOs on Jellicent and Sableye while in Aria forme, two Pokemon that block Relic Song. Its 60% paralysis rate (due to Serene Grace) is also a huge bonus to have, letting you give Jirachi that try to counter you a dose of its own medicine. However, if you prefer reliability, then Thunderbolt is a perfectly legitimate option. The last move is a toss-up. With a Life Orb equipped, Psychic is a powerful STAB move to hit many physically defensive Pokemon hard, while Hidden Power Ice lets Meloetta nail Gliscor and Dragon-types.Team Options & Additional Comments >>>
A purely physical set (with Relic Song, of course) that focuses on the Pirouette forme is plausible, but it is generally walled by physically bulky Pokemon while being outclassed by other physical attackers such as Terrakion. Her Aria forme would also be completely useless with such a set. A mixed set with Work Up or Hone Claws, coupled with Close Combat, Thunder, and Psychic also seems usable, but is outclassed by most mixed attackers in OU, and this is not to mention that the Relic mixed set does this just as well anyway.
Checks and Counters
Celebi probably stands out as the best all-around counter to Meloetta, resisting most of the moves that she carries, while not fearing paralysis due to Natural Cure. However, Celebi does need to watch out for U-turn. Latias with Substitute or Refresh is also a decent counter, although if she lacks Refresh, she must be very careful in switching into Thunder, lest she get paralyzed by it.
Otherwise, Relic mixed Meloetta can be surprisingly hard to counter; due to her forme-changing nature, it's almost like fighting two Pokemon at once. If she is in the right forme, she will generally be able to either hurt your switch-in extremely badly or switch to the other forme to keep momentum. The best thing to try and do would be to outpredict her and catch her in the wrong forme, although this is easier said than done. If you are able to get your bulky Ghost-type into her Pirouette forme or something such as Scizor into her Aria forme, you would have pretty much killed her momentum entirely. If you cannot do that, fast physical attackers such as Choice Scarf Salamence, Landorus, and Haxorus, can take down Meloetta quite easily, whichever forme she is in, while Pokemon with strong priority, such as Breloom, Scizor, and Dragonite, also do very well against both formes.