Years back I acquired a Craftsman manual impact tool, it has a 3/8” hex drive and will adapt to flat and Phillips screw driver or ½” sockets. You operate it by holding it against the screw or nut that you want to loosen and striking it’s backside with a hammer. I used it with a large flat screw driver bit to quite successfully to remove those fill plugs
By the way, once you get the shock’s end cover off you will find there is really nothing in there to rebuild. Just a couple of check valves to clean up and maybe a new gasket to cut out.
