This dissertation, conducted within the theoretical course-of-action framework, examines the activity of front-line humanitarian responders of a medical INGO, during the first phase of the COVID-19 pandemic. By better understanding their experience, the study aimed to contribute to the design of training environments to prepare humanitarian actors for complex unpredictable challenges, inherent in emergency response. We analysed the way a medical manager, in a central African context, dealt with the intractable problem to meet his hospital's oxygen needs, considered emblematic of the first wave's unprecedented resource scarcity. The concept of unruly problems helped identify the temporal complexity he faced. From a prism of resilience engineering, we found that a negative capability allowed him to navigate and tolerate the uncertainty, enact his environment, and use temporary solutions to create situational opportunities, until an innovative technical solution was found. Avenues for training design are proposed, according to a notion of Emergency Preparedness II.