Enemy is a very strong word. It's used when you hate/strong dislike someone. 敌人是一个很强的词。当您讨厌/强烈不喜欢某人时使用
Opponent is used in cases such as in a sporting match. 对手用于体育比赛等场合。
Rival is someone who is at the same level as you in a particular field. (Usually you aim to bypass this person) 竞争对手是与您在特定领域中处于同一水平的人。