Ask reflective questions. Limit yourself to rephrase your "opponent" views.
Listening like most have said here, is like the best tool.
Then take those questions unto conclusions... More than likely he won't agree with the conclusions, which will lead for him/her to sabotage her own theory.
Is no win/win though, mind your spirit. Debates should be used for entertainment purposes only. Know that you will not change nor redeem the other person and the more you prove your point the more likely she/he will grow apart from you.