I have Area Zero Underdepths in play, a Tera Pokemon-ex as my Active Pokemon, and 8 Pokemon on my Bench, none of which have the Tera Trait. My Opponent uses an attack that allows them to Knock Out my Active Tera Pokemon-ex and also two of my Benched Pokemon. Once my Tera Pokemon-ex has been Knocked Out and discarded, am I able to discard my two Benched Pokemon that have taken full damage before I count them as KO'd, or do I have to KO them first?

You have to KO them first and your opponent gets to take Prize cards for them. After they have been KO'd, you would then discard one final Pokemon from the bench due to Area Zero Underdepths and then choose a new Active Pokemon.

Relevant cards

SV7_131