Classic yin and yang, good versus evil.
Every character is significant; they all serve a purpose; they are all believable.
So you are writing from the protagonist’s point of view or antagonistic, either way, follow these two fundamental tips. There are other ways, though these two are the usual and classic ways to write up your hero versus villain.
First - Yin Yang.
Let’s say your hero is strong, fast, proficient with firearms, and martial arts, then your villain would be the complete opposite. He could be fragile, slow, but where he fails in martial, he thrives in intelligence and cunning. He is possibly rich, with an army of goons.
Both are pretty much equal, although they are complete opposites — a one-person army versus an army of goons.
Second - Superior.
Here you would write the main enemy as a force to be reckoned with. The threat should appear overwhelming, and your character should feel that they haven’t the slightest chance to win.
Maybe your character is an ordinary human with no specialized combat training, and the enemy is a powerful ancient vampire. Writing this kind of character makes the audience will your character on. It drives him to succeed, even though the tide is against him.
In this case, you could possibly use elements like the enemy underestimates him, or use the environment to your advantage to trap him. Your character would likely fight harder to, like a trapped rat.