Defending your thesis at your viva does not mean shielding it from harm or arguing aggressively.
(attack is not the best form of defence)
To defend your thesis well you need to know your thesis and research well. You need to know your field. You need to be a good, capable researcher. You need to know a little about your examiners. You need to have rehearsed for the viva with some kind of practice at responding to questions.
And all of these things are neither unreasonable to expect of you or too burdensome to satisfy during your PhD and prep time. You don’t need any aspect to be perfect.
To defend your thesis you need to do the work, be a good researcher and show up ready.
Defending your research doesn’t need to be more complicated than that.