"I am against him."
While using the android app, it enforced an answer of "ฉันไม่เห็นด้วยกับเค้า" via the tappable buttons. To me, I don't feel this equates to "I am against him" but equates more to "I disagree with him". I suppose you could imply that if you disagree with someone, you are also opposing them or are against them. But despite that connection, it still doesn't feel like a "best fit" translation. I prefer the main translation you provided of "ฉันต่อต้อนเขา", that feels like a much better fit to me.