if you have a gas axe(blow torch) use a centre punch while its glowing red to put a dint in the stud that wont budge as central as possible,use this to keep the drill from moving all over when attempting to drill it out,heat it up till it glowing again and try drilling it on a slow speed(so you dont get red hot swarf on you) using a smaller drill bit 3mm say,then reheat until red and go to a 4mm,i think the studs are 5mm,but im not certain
you might get lucky,ive done something similar on a snapped caliper bolt on my bmw that (ahem) was too rusted in to remove and i needed a new part!