Say the "Sky Field" stadium is in play and I have 8 Pokemon on my Bench. My opponent uses Salamence's "Shatter" attack to KO my Active Pokemon and discard the stadium. Do I promote one of my Benched Pokemon first, or do I reduce my bench size to 5 Pokemon before promoting?

When the Sky Field stadium is discarded during the attack, the bench size is reduced immediately before going any farther. So after promoting your new Active Pokemon there will be 4 Pokemon on your Bench.

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