Ok, you need something to lock or hold the prop hub adapter with then. Maybe a chain wrench would do it for you.
If you don't mind getting marks on the prop hub you could use a large vice grip or pipe wrench maybe too.
Some guys lock the crankshaft from the other side and then they can loosen the nut, but I tend to be reluctant to do that unless it is the only way. Some others use a piston locking tool inserted through the spark plug hole, but that has the risk of damaging the piston though.



The chain wrench Vice grips are pretty good for this too.