[Mithras]

From what you describe I think investing in cheap ways to prevent assisnations from occuring is a good idea. Dome all your mage concentrations, and places where you expect your mages to move to, (a couple of gems and 500 gold for a lab is cheap in the late game) and put your pd up in your fortress/have idle armies of chaff patrol. Its not a hundred percent effective but if you invest in vast amounts of pd all over your nation then you only have to worry about the elementals, which you can tailor script your mages for the earth elementals.