I haven't seen Meloetta very often, but when I do, it always surprises me just how much special bulk it has (before transforming, of course). It's relatively slow, but it can definitely take a special hit and strike back with a powerful STAB Psychic. It also has a decent movepool as well, sporting moves such as Psyshock, Thunderbolt, Grass Knot, and Shadow Ball on the special side, and Close Combat, Shadow Claw, Stone Edge, and U-turn on the physical side. Since Meloetta is hard-pressed to use its Pirouette Form, as it must use Relic Song in order to transform, I would think that either a mixed set or a full special set would be the most effective. It is also important to note that Relic Song is blocked by Ghost-types, though they usually cannot do much in return do to Meloetta's Normal typing making her immune to Ghost-type attacks.
Here's the set I'm thinking of:
Meloetta @ Life Orb / Leftovers
Ability: Serene Grace
EVs: 128 Atk / 128 SpA / 252 Spe (Not too sure about the EV spread, it's definitely tentative)
Nature: Naive / Hasty
- Relic Song
- Psychic
- Shadow Ball / Shadow Claw
- Close Combat
A full special set would probably look like:
Meloetta @ Life Orb / Leftovers
Ability: Serene Grace
EVs: 4 HP / 252 SpA / 252 Spe (Not too sure about the EV spread, it's definitely tentative)
Nature: Naive / Hasty
- Calm Mind
- Psychic / Psyshock
- Shadow Ball / Hyper Voice
- Focus Blast / Thunderbolt