"Él no es tu rival."
Translation:He is not your rival.
A rival is someone you are competing with. You could be shooting a target in a contest with your friend, and your friend would be a rival, if you both wanted to win. A rival does not have to be an enemy.
It is commonly used as a synonym, although in the correct use of the word it isn't
Hmmm... I wouldn't use it with same meaning. The sentence of the exercise probably means that "he" isn't in the opposite team, you are teammates, but your sentence says that "he" can't defeat you. In Spanish, it's "Él no es (un) rival para ti".