My opponent used Perish song when I had two mons left to his four. I swept all of them on the last turn and immediately after fainting his last mon, the counter hit zero and my last mon (only one remaining) fainted from perish song. I got the loss. This is frustrating to think I beat it and lose anyways, so if someone could explain it better it would be much appreciated.
Edit: Perish song was used when I had two left and he had four. I swept all four and finished the fourth on the last turn of Perish Song (counter reached 0 same turn). I believe Amoongus was my last mon and Pollen Puff was the finishing move used in trick room.
Only one Pokemon fainted from perish song.
Please do not say stuff like "When multiple Pokemon faint from perish song, the fastest one faints first."