Primary: constitution Secondary: wisdom Tertiary: strength
This skill is automatically in use.
The ones with limited ability usually make up for their poor skills with determination and guts. However, determination is not the sign of a poor warrior, on the contrary, and a healthy, strong minded combatant can recover from being stunned a lot faster than a weak willed one, perhaps even shake it off completely. Self-motivation and concentration are important, and also learnable, ways to achieve this state of mind.