Electivire Lv. X vs. Buffer Piece

[SuperWooper]

Electivire Lv. X uses Pulse Barrier against a Pokemon with Buffer Piece attached. Which comes first; the 50 damage or the discarding of the Buffer Piece (resulting in only 30 damage done)?

 

[PokePop]

You have it backwards.
The damage comes first, resulting in 30 damage.