I've been using this set:
Meloetta @ Expert Belt
Ability: Serene Grace
EVs: 64 Atk / 252 SpA / 192 Spe
Nature: Hasty
- Relic Song
- Close Combat
- Thunderbolt
- Hidden Power Ice
Basically, the idea of the set is to set up multiple layers of hazards, and then spam Relic Song to force switches. Counters to Meloetta-A tend to be weak to Meloetta-P (Blissey, Snorlax, Tyranitar), while counters to Meloetta-P tend to be weak to Meloetta-A (Skarmory, Gliscor). The idea is to spam Relic Song and keep changing forms and stay one step ahead of the opposition. With these games going on, you can probably HP Ice a Gliscor or something like that and secure a sweep for Excadrill and stuff. With Fighting resists removed, Meloetta-P can handily clean up teams by itself as well.
EDIT: Oh yea, the EVs used let you outspeed Adamant Lucario as Meloetta-A and Alakazam as Meloetta-P.