Originally Posted by 97 SL320

Take a solid drift ( like the input shaft from a Ford C-4 or C-6 transmission since they just pull out of the front ) and hammer on the hold head. This will reform the stripped section a bit and imprint the underside of bolt head into the attached part. Doing this will relieve some bolt tension. If you can hit the head sideways that can help also.

Make sure new and sharp the allen bit you are using is ground flat on the end, some have a chamfer that will reduce grip. Try to use a T handle breaker bar, when using two hands you are applying pure torque rather than torque + bending moment that leads to the bit coming out. If this isn't possible, support the ratchet head / breaker bar to contain the bending moment.