IIRC thats a 8mm allen.
is it the front bolt that also secures the ABS sensor, or (hopefully) the rear bolt?
use a drift/punch with a flat end, striking it with a hammer against the top edge of the bolt to upset the metal and cause the internal hex to expand inward. Work carefully around the other edge with the drift and hammer.
done correctly, your 8mm allen socket will no longer fit inside the bolt, line it up properly, then knock it into the hole with your hammer.
use a 6-8" extension on your socket, one hand on the ext. and the other on the ratchet, apply controlled pressure against your ratchet, if it feels like it is going to strip again- STOP
heat can be your friend here if you have a heat gun or have access to a small ox/acytelene torch. Spray the bolt with WD40 or similar, and heat until it starts to smoke, spray/heat several cycles, then repeat the hammer and punch treatment.
with some luck and patience you will get it.....
Triple M Engineering