I think it's level up. Level up Shuckle holding a Berry and it will turn into Berry Juice (at least that is what I read on how to get Berry Juice). Use the Bait and Switch Method so that Shuckle doesn't use it's Berry.
To my experience, just winning battles (random encounters) with shuckle in your party will sometimes transform shuckle's held berry into a berry juice. I also read that the berry juice will eventually be transformed into a rare candy, can anyone confirm this?