So Echoed Voice turns into Fairy with Pixilate, gaining STAB and 1.3x power, and used consecutively while holding a Metronome (which can get up to a 2x multiplier, let's just say Sylveon survives that long), what's the BP? It's gotta be crazy high.
Echoed Voice deals damage. Its base power increases by 40 each consecutive turn it is used but reaches a maximum power at 200.
200 (Echoed Voice ) x 2 (Metronome ) x 1.5 (STAB) x Pixilate (x 1.3) = 780 power.