That nut is a pain to get it out if not done with the correct tool.
When I did the timming belt I had to remove that nut to remove the alternator belt, and had to buy a good combination wrench, and use the "closed" part of the wrench, as in this one:
The problem is that one can not get a socket wrench, its quite tight, even with a 1/4" dive socket I was not able to take it out...