King Piccolo is a skilled enemy from the classic Dragon Ball days. That being said, his power level has long been surpassed by the heroes and I don’t believe that he would prove to be much of a challenge for Slug. Slug went up against Goku and the Saiyan’s Kaio-Ken technique. Goku never needed such a great tool against King Piccolo and I’d say that really seals the deal. Slug wins.