Talk to the hiring manager about how you are able to maintain a level head during stressful situations. Give an example of positive feedback you have received on your ability to handle stress.
"I handle stressful situations very well. Whenever I feel a bit overwhelmed, I will take a step back for a breather. I try to re-assess the situation and look at it in a new light. My current manager comments quite often on my ability to handle high pressure very well."