I've been working on a game for a while and think that my fights might be too easy, as a general rule, would it be better to nerf the MC or to buff the enemies?

Does MC mean "main character"? You should both nerf one and strengthen the other, so that they meet in a happy place between too easy and too difficult. 

With absolutely no info given your question is akin to asking if you should make the sky bluer or the clouds whiter.