Reductio ad absurdum
Reductio ad absurdum is a type of logical argument where one assumes a claim for the sake of argument, arrives at an absurd result, and then concludes that the original assumption must have been wrong, since it led to this absurd result. Note that this is a logically valid technique.
See Can God create a rock so heavy that he can't lift it? for an example in the context of counter-apologetics (the claim being assumed is that God is all-powerful).