I have undone lots of viscous fans. I used to look after a fleet of Transit vans, the old smiley face ones that had a timing belt, the hammer and chisel worked every time and never damaged the fan, hub or nut. You just need to position it on the edge of the nut and give it a very sharp and hard strike. I promise it will come undone. I fitted a new viscous hub to my Rangie and it had done around 140,000 at the time and had spent a big chunk of it life on the Shetland isle before I had it so it was never far from the salty sea air. One sharp strike with a cold chisel and it was undone. Do not wedge the chisel in it needs to be the same width or just a few mm smaller than the depth of the nut as it needs to be a nice even blow across the depth of the nut. I hope that makes sense.