Can be a bit of a pain to get out, worst case is your taking the head off.
I'd douse it in WD40, get out your Torx sockets, find one that fits in the hole tightly, then go up a size (Should be about T50) then bash this on STRAIGHT with a hammer & an extension, try to undo it like that.
I take it you can see into the cylinder? as in the electrode bit has come out, the white bit?