Haha oh man, you're actually on the wrong side of the wall to get the Duck, You can get to him after you encounter the boss! Look for the skull that will show you the way!
I got the duck then lost all my lives. Got the duck, lost all lives. Got the duck 3rd time and finally got to listen to everything it says. Now back to getting all skulls.
Haha we've all been there, I'm gonna guess which one you're missing and give you this hint! Near the comets, they'll give you a lift to your prize, but skewer your plans if you're under them.