The first method would use the intended tool, a fixed cup wrench. If only the low cost flat stamped "box" type wrench is available some way to secure it to the shell will be needed. But if you have access to (like a better shop will have) a Var/Campy type of vice tool all the better. Only when this vastly better fixed cup tool isn't the correct size has this method failed me. The tool can be held in a large bench vise to make the frame the lever arm.
Next up would be the Sheldon method which is borrowed from the Kingsbridge type of tool (Bicycle Research made a smaller version too). Using the specific to BB cups tool, and not generic bolt and washers, helps a lot. I've removed a number of fixed cups that customers brought in after the Sheldon method didn't work and my Kingsbridge tool did. Again using that bench vise is a big aspect and how you hold VS turn which side of the BB shell you're on is important
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At any point along this process soaking with a good penetrant, like Kroil and not WD40, can help but isn't a guaranty of an easier effort.
When all else fails there's heat or mechanical erosion methods. heat means well more then boiling water levels, think burning paint levels. And with the mass of steel this will take a powerful heat source applied for a while. Errosion usually means grinding with a die grinder or Dremel. At least until the cup's threaded wall is thin enough to see the thread tips. This only needs to be done in two locations, roughly diametrically across from each other. Then a careful application of a cold steel chisel against the two "halves" of the cup can break them free of the shell's thread. Of course if you have access to a big EDM machine that would work too
The images suggest that the OP hasn't tried much, if at all, with a fixed cup wrench. Is this true? Why not? Do you have a LBS near by that you can work with? With no location listed in your profile I don't know if this option is easily available to you. Andy