Blackbeard needed Koby's and Law's help, to kill the former ruler of Pirate Island. I doubt this was because Ochoku was so strong. It was probably just hard to get your fingers on him. Likely he had a very useful Devilfruit, that Blackbeard gave to one of his Titanic Captains. My number one guess would be Augur's Warpfruit. But Pizarro's makes sense aswell. Ochoku could have ruled Pirate Island quite effectively with it.