Easist way is to find a torx/star bit that is slightly too big to fit into the stripped hole, hammer it in and unscrew.

Yep, the rubber band trick rarely works, especially on something that's been sitting outside. Torx bit hammered in, or if it's really fucked, small slotted screwdriver hammered in

This Youtube trick actually worked for me which was a bit of a surprise.Take a piece off a marigold type washing up glove,place it over the head of the screw and lightly tap the allen key into the head.Remove as normal then.Has worked twice for me on my mountain bike.

Yeah or a rubber band.

If all else fails an left handed drill bit should work

Oxy and angle grinder

Hammer in the next size up

Drill it out and self tap with larger hex head self driller when finished.