In the room right after you get Coo, there's a wall with a slope and an umbrella enemy. I don't remember exactly what I was doing, but I'm pretty sure I was moving forward and mashing A while inhaling the Umbrella.
Yeah, that place. Fff... I'm trying to remember the details since it was a year ago. I believe I went underneath the slope as I inhaled. Coo wasn't "equipped" since I couldn't press down to swallow.