Are you using a proper headset cup remover ?
Then get a bigger hammer
Might want to soak it first in a solution of thinners or a penetrating oil. something to help weaken the grip the corrosion between the cup and headtube. Plastic container to dunk it in then leave it overnight.
Love watching the fear on customers faces as you use brute force to free something. I have bashed the utter hell out of things like BB cups and im sure i must have been applying massive forces with extended scaffold poles on the end of spanners. Same with removing pedals, we've have 3 people on the end of a big extended wrench undoing a pedal. You'd have thought the crank arm would have snapped but it was the corrosion that went first.
A proper tool will hold itself in place. You need to hit it squarely, thats about your only worry, and even then i dont really mean 'worry' more be aware
If you really arent comfortable walloping it a last resort would be to carefully mill away one part from another.
About all i can think of sorry, but i would stress having it dunked in a penetrating solution first before making further attempts.
Theres a very good pro solvent for this. Apparently eats alloy oxide but not the alloy itself and is recommended for seatposts. I really cannot remmeber the bloody name
1976 Raleigh Chipper,1991 Muddy Fox Adventurer, 1992 Raleigh Diablo ,1992 Rudy Project, 1995 Cannondale M200 ,1997 Santa Cruz Tazmon ,1997 Specialized Stumpjumper ,1997 Specialized Rockhopper ,1997 Marin Quake 9.0,1998 Marin Pallisade trail ,1998 Raleigh Torus,1999 Orange Clockwork , 2005 Kona Hahanna , 2010 Orange Pure7
The Marvellous Mechanical Hope Collection