General:
* Adversary: This is a formal word for an opponent or enemy.
* Foe: This is a more poetic word for an enemy.
* Opponent: This is a more neutral word for someone who is competing against you.
* Rival: This is someone who is competing with you for the same thing.
* Nemesis: This is a powerful and dangerous enemy that you are destined to fight.
More specific:
* Antagonist: This is the main opponent in a story or conflict.
* Villain: This is a character who is evil and harmful.
* Predator: This is someone who preys on others.
* Detractor: This is someone who criticizes you unfairly.
* Oppressor: This is someone who abuses their power over others.
Informal:
* Bad guy: This is a casual term for someone who is evil.
* Foe: This is a more informal word for an enemy.
* Frenemy: This is someone who pretends to be your friend but is actually your enemy.
The best synonym for "enemy" will depend on the specific situation.