Axle casing problems

Anyone got any suggestions on how to remove a damaged nut from the end of the axle casing. The axle has split on mine and i have got a new casing but one of the seven bolts that hold the half-shaft in is knackered. It is impossible to get at them with a socket, only a spanner will fit and the head was rounded on one when I started, it is driving me nuts (scuse the pun) as this is the replacement casing I haven't started removing the axle from the car yet. thanks

drill the head of the thing off then once you have got the rest out the way a pair of mole grips should do it...if you need to remove the busted bit