Common problem, people do them up too tight and then use a poor-fitting tool to undo them. They have a tapered thread, so tighten as they go in to make the seal. A 12mm hex key is only about 4" long and allows ample tightening torque.
New plugs are cheap.
1. Try grip pliers.
2. Weld a bolt into it, then use two nuts on the bolt to undo it. Throw it away and replace with new.
Find a suitable bolt. Original internal hex is 12mm, so a 13mm head will not fit, but could grind/file it down. Do the preparation work yourself. Then a garage should be able to splash the weld on.
3. Could try drilling it and using an Eezi-out, but that may not be successful, and then you've got a hole. Drilling swarf will come out with the old oil as long as engine not run.