I smiled when I saw the above XKCD cartoon on Facebook. But from a more serious side, despite the saying that “the best defense is a good offense,” that isn't true in the world of arguments and reasoning, whatever its validity may or may not be in the realm of sports.
No matter how much you attack, attempt to poke holes in, or argue against a viewpoint different than your own, it does not make your viewpoint correct.
If you have a thesis – whether in the sense of a written dissertation or in the sense of a viewpoint you have been developing in your mind – the question of its truthfulness or its usefulness in making sense of the world must be determined by assessing its fit to the evidence.
If it doesn't fit the evidence, and someone points this out, it is no good attacking alternative views. It may turn out that those other views are wrong in some way, shape, or form. It is likely that all human viewpoints are to a significant extent. But you viewpoint may be even more wrong than those you are attacking.
And so by all means relax yourself when you have a thesis defense by imagining yourself chasing the examiners with a sword.
But when considering things more seriously, know that a solid thesis must have as good a defense as an offense. If there is nothing to criticize about other viewpoints, then you will probably have nothing insightful to contribute. But in order to have a valid thesis, your own viewpoint must withstand criticism better than those other viewpoints do. Otherwise you have (as should be obvious) made no progress towards getting closer to the truth. You have just added one more problematic viewpoint – and perhaps a more problematic viewpoint – alongside others.