Did you use a good quality screwdriver? We've found cheap screwdrivers aren't perfect so if there's a slight issue with the screw it'll cause damage.
Only suggestion I can think of is to get a screwdriver that has a spanner flange on it, push as hard as you can to keep it nicely located in the screw and get a mate to turn the screwdriver with a spanner.
If you don't know what I mean about the screwdriver that probably won't make much sense!
We've undone worse looking screws like that with this method.