Difficult in removing screw inside shower
Knob

Difficult in removing screw inside shower
knob

Penetrating oil is your friend... also after soaking it a day or two... put the screwdriver or Allen wrench into the screw and tap it lightly with a hammer... If it resists... squirt it a time or two more with penetrating oil and wait another couple days.

After trying all other reasonable approaches, drill out the head of the screw, remove the handle and hope that enough of the screw protrudes so that you can grasp it with a pair of vice grip plyers.