This is why you can't trust anyone to work on your car.

Yeah a sharp chisel and hammer usually makes them turn. The tension is at that flanged portion of the plug.
Several years ago a guy calls me and says he cant get an oil filter off. He was no mechanic, just trying to make points with a new girl friend. I forget what POS lil ride it was but the tiny oil filter faced the firewall in close quarters. The guy had already drove a huge screw driver through it and ripped it all to hell. Cut a finger too. Oily freakin' mess in this gals carport. Anyway...
I have this plumbers tool that is simply a loop of heavy strap attached to a piece of square tubing. It is a bit of a challenge to operate because you have to tighten and then hold it there with a second tool so it doesn't back off while you're getting another bite position to tighten more. This tool has never failed to remove the most stubborn filter including the remnants of one recalled here. Important tip to using this type tool... position its strap as squarely and as close to the engine block as possible.