Quote Originally Posted by Bisquit
I think it's better to say "оппонент" instead of "соперник". "Соперник" is generally used to denote a rival in sports and "оппонент" is used to describe a rival in scientific dispute.
good point, although if for example you're having a debate contest and you want to emphasize the contest aspect of your debate while talking about your opponent then you could use соперник here. Bottom line is you can't really say whether соперник fits in this sentence or not without more context, althoug it has to be conceded that if you were to google for "дискредитировал мой источник" the majority of the hits would probably have опонент rather than соперник in the same sentence or paragraph.