you can take a pair of locking pliers (use good quality vise grips or something like that, and don't be tempted to use needle nose visegrips), the biggest pair you can get to fit, and insert the visegrips into the circular shroud. its called parrot beaking. the vise grips will stick straight out from the shroud (long axis of the pliers parallel to the axis of rotation of the valve. once you have a grip, and the size of the plier is important here - smaller pliers will fit further in, but larger ones tend to hold better for the next step. you have to find a compromise. once the pliers are in there and clamped hard, you place a screwdriver or drift punch through the pliers jaws (behind where its clamped to the valve) and apply torque. just go slow and watch for bending of the jaws away from each other. you dont want the punch to lever the jaws apart to the point they slip off the brass flats or worse, round them over. also be wary of rotation. some valve bodies have right hand threads, some left.