Pickle is a Hanma ancestor.
Self explanatory.They could have just filled the arena with water and fed Pickle hippos. Hippos are hyper aggressive, fearless, heavily muscled tanks. They'd have been perfect prey for Pickle as they'd have gone after him relentlessly and aggressively. Don't fed him predator who have to weigh risk/reward in their fights. Feed him HERDS of hyper aggressive herbivores.
Pickle is a relict synapsid that coincidentally resembles a modern human.
It would explain how he was alive during the Mesozoic era and how weird his biology is.