Can you get the fixed cup out ? If so, remove the axle, then try big channel lock or big adjustable wrench on those flats.

If that doesn't work, then you can use threaded rod and bolts to rig up a better "grab" on the cup, same as the method for removing stuck fixed cups. For example you can then clamp an adjustable wrench to the cup so it doesn't slip when you apply more force.

PS, make sure you are turning the correct direction. Adjustable cup should be normal counter-clockwise to remove.