There's a real risk of tearing the immersion boss out of the cylinder if you get too violent with it!
Heating the immersion with a blowtorch to soften any sealant is one way.
Drilling a hole in it, then cutting slots from the hole to the edges of the threads so it can collapse inwards a bit when you apply a spanner is another.
Using the correct tools would be my suggestion.Apply heat and either use large enough grips or better still the correct box spanner and apply force evenly.
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