Don't attack strength You don't try to kill every stone of your opponent. You only need a few points more in the end than your opponent. In discussions that translates into accepting some premises of your opponent for the sake of the argument.

Take the evolution debate. You probably won't convince a hard core Christian that God doesn't exist. If you however admit God's existence for the sake of the argument you can use it to make strong arguments:

The bible shows that God prefers evolutionary design over intelligent design. God ordered a flood to kill the majority of the human race because he selected some humans as having better traits then the other humans. God choose the Jewish people and ordered them to commit genocide on other human groups that he didn't like.

Seeing God's genocidal ambitions as his attempt of evolutionary design. Otherwise the morality of killing that many people is hard to explain and the Christian you are arguing with has the burden of providing a better justification for God's attempt at genocide.