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Pirouette Forme only lasts until Meloetta is switched out after using Relic Song in battle first.
They could've at least made it last till the end of the battle.
They also could've made an item that transforms Meloetta into its Pirouette form and back. I mean, if they can do something like that for Shaymin, why not for Meloetta?
I'm not big on forms that require taking damage or using a certain move to activate in general. It's just too unreliable.
Darmanitan's Zen Mode would be another example.