Stalling at +-90 degree bank angle, 30 feets above ground. I don't think you can do it without both luck and skill.
When I started flying, someone told me "you start with a empty bag of experience and a bag of luck which you don't know how much is in. You want to fill that experience bag before the luck bag is empty". I guess Matt Hall just find out there was left overs in his bag of luck.