One of the head bolts on my 1997 Mercedes C230 stripped during removal. It is a 12 point star, I think they made be called triple square splines. The others are about 4 inches long stretch bolts. There isn't enough space to get anything in there.
Some suggestions I have heard so far and have tried some are hammering a larger size into it, using an impact driver, and welding a socket onto it to give it something to grab. Keep in mind its an aluminum block.
A mechanic friend of mine offered to do an engine swap and since the engine already had high miles I just opted to do that.