In the past I have drilled holes down throught the bolts prior to attempting to undo them as it is easier to centre the drill in the hex hole than trying to centre it on a sheared bolt.
One method worth trying is welding a nut on the sheared bolt.Getting steel (or stainless) screws out of ally is always a ball-ache anyhow, plus they they're Loctited in.It's a galvanic reaction between two dissimilar materials that cause the seizure.One common error when tightening bolts is to use ratchets etc that are far bigger than need be for the job in hand (compare an Allen key with a socket ratchet / bar for length and you'll see what I mean) - even a long Allen key can apply far more force than is often realised.