"I am against her."
Translation:Ich bin gegen sie.
"Gegen" is a preposition here, not a verb. Some German prepositions follow the object, but this isn't one of them:
Some linguists call "prepostions" that come after the object "postpostions", but "preposition" is still the most common term for both of these kinds of words in English:
If you go to: https://www.duolingo.com/skill/de/Accusative-Pronouns and scroll down to "Tips and notes" it gives a list of pronouns in the accusative case.
The bottom line is that sie (she), sie (they) and Sie (you formal) do not change in the accusative. It definitely makes it confusing! So the sample sentence, "Ich bin gegen sie" Could mean either, "I am against her" or I am against them."
No, "kontra" does not work here. Best translation of "kontra" might be the English "versus", but you do not really use "kontra" very often in German. The one exception is the saying "Pro und Kontra" (the for and against i.e. advantages and disadvantages), which is quite popular.