Sly is a small mini-boss in Lyre, Lyre pants on fire. Once you follow Lenny to the end of the path you enter the building to find Sky waiting for you. If you have the heat seeking mega rang the. You will have no issues with this fight. All you need to do is hit salt with rangs to hurt him. The best time to do so is when he stops and winds up for a throw of his rang. This fight is more of an endurance test than anything. Once he is defeated the achievement pops.