If the bolt was seized enough to allow twisting of the head off, no slot this or walk out that is going to work, sorry.
Question, which bolt is this, one of the short ones that terminates in the timing cover or is it one of the long ones that goes into the block? If it is the latter, you will be taking the timing cover off to get it out, sorry to say it is pretty much the only way. If it is one that teminates in the timing cover, you might have some luck using heat if you have a way to get a good grip on what's left of it.
Also, did it come out at all or twisted off right from the get go? If the former, you might do well to work on screwing it back in and work it from where it was able to move some.