It is possible that someone used Loc-Tite on that screw. Let a soldering iron get as hot as it can, then hold it to the tiny screw for a bit, then attempt to loosen.
Soldering iron not hot enough? Use propane torch to heat a nail or iron bar red hot, hold red hot against the screw for a bit, attempt to loosen.
Soldering iron not hot enough? Use propane torch to heat a nail or iron bar red hot, hold red hot against the screw for a bit, attempt to loosen.