"Eu sunt contra ei."

Translation: I am against her.

[emeacock]

Would the translation of 'I am against them' be different?

[coto.i]

I am against me. = Eu sunt contra mea.

I am against you. = Eu sunt contra ta.

I am against him/her. = Eu sunt contra lui/ei.

I am against us. = Eu sunt contra noastră.

I am against you. = Eu sunt contra voastră.

I am against them. = Eu sunt contra lor.

[-masteRice-]

Okay, but why isn't it "Eu sunt contra ea"?

[daev3000]

Because the object of the preposition "contra" takes the dative form.