Clayton is a boss fought in Deep Jungle in Kingdom Hearts. Sora and company confront him just as he is about to gun down a group of gorillas he had gathered together. Clayton faces Sora in battle twice, first flanked by Powerwilds, and then partnered with the Stealth Sneak immediately afterwards.
In the first stage of the battle, Clayton's only weapon is a shotgun, good for distance attacks, but he also fights alongside a bunch of Powerwilds. Ignore them and lock onto Clayton, bashing him until his HP is about halfway down, but be warned halfway through his life he will use potions. At this point, the second portion of the battle begins.
When up against Clayton for the second time, the cliff walls explode and Clayton mounts an invisible creature, the Stealth Sneak. His only weapon is still a shotgun, which is easily reflected and does not do much damage regardless of being mounted on the Stealth Sneak, so Clayton himself actually poses no true threat, so focus on the Stealth Sneak. Dispose of this creature first, then find Clayton and bash away at him. You will have hit the Stealth Sneak a certain number of times to knock Clayton off in order to attack him. Your prize for winning is the Cure spell.
Clayton may heal himself in the middle of a battle. But still, his only way to attack is by using his shotgun. All you need to do to deflect it is to hit it right back at him, or perhaps use the Guard ability, considering you have the right circumstances to have acquired it at this point. Note that even in the battle with Stealth Sneak, once you defeat Clayton, the battle ends, but you can still defeat the Stealth Sneak for extra experience.