Alneyan is right about the magic resistance stuff AFAIK though I believe the basic penetration is 11, not 10.
Indeed I'm almost certain Gandalf does have a few such maps that provide additional challenge depending upon who you select to play. Orania - nasty edition for example I think one is called.
If you want more challenge from you AI experience however for any map, I have had great success over the last while with some utilities I wrote. Since I felt there were others out there like me who enjoyed a game against the AI once in a while, I published them in this
thread.
Look at utility 1, called GenMM. It is easy to use and should allow you to provide yourself with as big a handicap as needed.