With the freewheel side spokes missing all I can say is "good luck". Fietsbob is right in that the best way is to partially rebuild the wheel to get the leverage. But it'll have to be done on just the one exposed side. The only good news is that since it's just to allow spinning off the freewheel that they can use all the spokes angled in one direction.
The only other option is to do what I did with some freehub hubs and make up a wooden clamping caul that has holes sized to be just a hair smaller than the middle of the hub. You drill these holes through the block of HARDWOOD such as maple, cherry, birch or similar and then cut it in half lengthwise through the hole so you can grab the hub between the two halves with a good stout vise. Normal softwoods for building won't work. It has to be a good hardwood to avoid just deforming and letting the hub spin.