Raise the leg so its at a decent height to work on. Also ensure the screwdriver blade fits the cross head screw. Stanley or Draper make decent rubber handled screwdrivers with hardened blades, usually just over a foot long. More often than not the wrong size screwdriver is used and it ruins the head of the screw.
If you have locking long nose pliers then clamp a set onto the rubber or plastic handle of the screwdriver. Leaning into the screwdriver, turn it and lever the pliers at the same time. Job done. Renew the drain gasket once it comes off.