'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Clamp the freewheel removal tool to the freewheel using the qr skewer or the axle nut to assure it doesn't slip. Those older 2-prong Sun Tours freewheels were easy to damage unless the tools is held firmly in place.
Then use a BIG wrench on the tool flats (I used to use a 15" adjustable) or, as noted, clamp the tool in a bench vise and use the wheel itself as a the lever. These things can get cranked on super tight and, along with corrosion and neglect can be very difficult to break loose.
As SOON as the freewheel breaks free, begin to loosen the qr or axle nut to let it unthread.
Use the quick release and a vise. It's easy and non-destructive that way.
Or use a big wrench, if you absolutely have no access to a vice. With a wrench, there's risk of at least skinned knuckles and foul-mouthed expletives, and some risk of damage to the wheel--especially if you decide to bang on the wrench.