Use the correct-size Allen key and apply firm downward pressure
Tap the Allen key into the screw head with a hammer
Spray penetrating oil and let it soak
Insert a rubber band between the key and the screw head
Try a slightly larger Torx bit hammered into the stripped socket
Use locking pliers if the screw head is exposed
Cut a slot in the head with a rotary tool and use a flathead screwdriver
Apply heat to the surrounding metal if appropriate
Use a screw extractor kit
Drill out the screw head
Remove the remaining screw shank with pliers after the head is off
Replace the screw with a new one after removal