The Side A Virgo Riddle requirement is as follows -
"Dealt a KO! with only one button push".
Most KO's on earlier bosses are performed by button mashing, followed by a single button press after the mashing. You can ignore the prompt of button mashing, waiting until the prompt to do the final button press. Performing just the final button press should still result in a KO, just not as flashy, and also give you this trophy.
The one I got this on was the boss at the end of "- The World We Knew -". After some minigames within the bosses world, I only had to press one single button to defeat him, popping this trophy.