Hold the gun in a padded vice .Use only correctly ground screw drives ,I don't use turn screws never have and never will .
Penetrating oil,heat ,cold what ever but a good size screw driver handel that you can get good grip on is half the battle in my opinion . If its a flat bladed driver then a spanner can give you the extra torque needed .Please also remember that some guns have hidden locking screws on cocking dog screws and top lever pins for example .