Slimes are the weakest enemies in the entire game, being one of few enemies that can be instantly killed by the slingshot. They're seen in various locations of the game, mainly in forested areas like Duri Forest and Misty Gorge.
When you first encounter a Slime, it'll be in the form of a puddle on the floor or ceiling. When Gale gets close to a Slime, it'll wake up and look around for a bit before chasing after Gale for a few seconds. All you have to do is whack it with your weapon when it gets close enough.