ChickenHeart's pretty much nailed it. The only I'd add to that is, despite several sources on the internet claiming otherwise, you can leave Centra Ruins to save and do other things when you're hunting Tonberrys. This is useful because you'd want to save after defeating the 16th or 17th Tonberry. The King appears immediately after you kill the 18th-22nd Tonberry, so you won't have the luxury of saving and healing up before you fight it. People have gone into the Ruins thinking they can't leave until they've killed all the Tonberries and lose to the King when he unexpectantly pops up, meaning they'd have to start all over again.

Degenerator works well on them if you're wanting to kill them fast.