Use the correct-size Allen key and press it in firmly
Tap the key into the screw head with a hammer
Apply penetrating oil and wait
Use a rubber band or steel wool for extra grip
Try a slightly larger Torx bit hammered into the head
Use locking pliers if any part of the screw head is exposed
Heat the screw to loosen threadlocker
Use a manual impact driver
Cut a slot in the head with a rotary tool and use a flathead screwdriver
Use a screw extractor kit
Drill out the screw if all else fails
Replace the screw after removal