I believe that good combination would be training, knowledge , experience and mental state ( life philosophy or similar)

While now I have all of the above,I remember the time when I did not have anything, and first thing that I actually used was something like mental state, or you could call it philosophy of acceptance.
It helped me much, until I learned other things.

You need to have “cleared” state of mind in hard times, in order to avoid all s..t that can pull you and get you killed.

Do not get me wrong, having mental toughness alone does not mean too much, but it will help you a lot to put everything else in correct order.