I would just keep moving it back and forth with a lot of liquid wrench sprayed on the threads. Maybe a little heat from a torch?

On new plugs, especially with alum cylinder heads, I always apply a little bit of thread anti-seize compound to the spark plugs, not too much. I have never had a problem getting the old ones out. (Of course do NOT over torque)

Good Luck.

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1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)