Nothing more frustrating...

Are you using a socket wrench, or would that not fit?

If you are using an 'L' wrench, you can put a 3/8" steel pipe on it to get more torque, which most hardware stores have in various lengths.
Sometimes you can put a pair of vise-grips on the end, or even a crescent wrench.
Both of these solutions presume you have the space.

Have you tried percussion? If you can get anything in there to whack on the L wrench, that might work.

Sometimes you can use a pry bar. Get the arm of the L wrench close to the head/cover, slide prybar between until lower end is sitting on the head, and pry. May not be room.

Hope one of these helps.

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1986 300E 5-Speed 240k mi.