You are going to have to drill it out. If there is no way to grab hold of it from the outside, that is about your only alternative.
Use a cobalt drill bit - try a left-handed bit (availiabe in good hardware or some auto parts places) they sometimes will back out the stud while drilling.
If they don't, you can next try an easy out - but go easy - you don't want to break one of them off in there - they are majorly hard and impossible to drill. If it feels like the easy out is not getting it done, drill it all the way - and for the correct size - then re-tap the threads. As a last resort, if the hole is really buggered - you can go up in size one size (might have to ream out the WP boss) and/or insert a thread repair kit.
