When you want to say "vs." as in the sentence above you always use εναντίον. In other sentences you can use either "κατά+Genitive", "εναντίον+Genitive" or "ενάντια σε+ accusative". P.e: I am against this decision can be "είμαι κατά αυτής της απόφασης", "είμαι εναντίον αυτής της απόφασης" or "είμαι ενάντια σε αυτήν την απόφαση. If against has a meaning like "sail against the wind" only εναντίον/ενάντια is used.
There is no context (unless one knows this is a sports reference? Which I did not)? Also, since it has happened before why are there hints/suggestings upon hover that aren't correct? "as opposed to" was the first hint listed. With no other context or understanding to go off of, how would one know that it is not correct then?
If the hint was listed first, it was wrongly listed. Εναντίον does not translate to as opposed to. Αντίθετα does, σε αντίθεση με does too, perhaps αντιθέτως as well. But they are not interchangeable with εναντίον, and since that is the word you are required to translate, not everything fits as a translation for it.