If it is really stuck you aren't getting it off with a home remover tool. Go to a bike shop that has been in business since those freewheels were common. The Campagnolo remover that came with the full Campagnolo tool case does the job easily and quickly and doesn't damage the (unfortunately fragile) notches. The Campy tool will usually get it off even if the notches are already damaged. There are a few Campy-copy tools that do the same thing.

If it is well and truly stuck you start disassembling the thing and then a pipe wrench. You can end up destroying both the freewheel and the hub. Again, if you are in this bad situation it is best to take the job to a shop and a mechanic that has done this before.

I have these freewheels on all my bikes and have preferred them for 50 years. There is a way to avoid the problem. Looking at freewheels that were really a bad problem, the ones that did require a pipe wrench, that did cut new threads into the hub shell, they were always heavily greased. The ones assembled dry or with a few drops of oil always come off easily. Yes, anyone here can provide cites to the usual respected authorities that will tell you to grease freewheel threads. Regina is a lot easier to live with assembled dry.