The only problem with this method is that it is easy to make the crank 'out of true'. All the load that you are putting on the flywheel nut is being transferred along the crank with the distinct possibility of revolving (slightly) one or more of the journals.
If you intend to have a crank rebuild then it's possibly ok but a flywheel holding tool is by far the best (maybe only!!) method.

Mike
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I don't mind dying...I just don't want to be there when it happens!