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The IGN walkthrough doesn't say you NEED it, it just implies that it'll make the battle a lot easier. The medal grants immunity from all ailments except unconsciousness. You can use other items to protect from or recover from specific ailments.
Many locations can be visited long before the story tells you to go there. Some might be dangerous due to tougher beasties, but don't forget that Swaine can steal items from enemies, and you can flee a battle before you're killed. Even if you're killed, I think paying the 10% fee to continue will let you keep anything you stole during the battle.