Once you arrive here, head straight for the camp site and eat/sleep. You'll wake up and have your conversation interrupted by a creature who stole Kay's pants. Now the chase begins! Chase after it to recover your pants until Kay thinks up of a way to propel Yu to catch the creature. Once you've recovered the pants and learned how to propel your partner,Katepa islet
. Throw your partner towards the Flow-grubbers as they break right/left to make your partner hit the dirt face first. Once you get both Kay and Yu to hit the dirt, the achievement will unlock as one of them says something along the line of "I got you back!".