[Mithras]

Don't forget about patrolling and magic domes. Patrolling and high pd should clear out mundane assinations pretty well, especially if as you imply the mages that are getting assinated are with big armies. If your moving your armies the assisnations shouldn't be able to keep up, if your staying in one province then you might as well use that big army for something. If you don't have a big army then unscript the army wide buffs for something better at taking out assasins.

If its spell assasins that are bothering you then invest in some domes. Go for one that absorbs multiple hits or strikes back at the enemy, assasin spells are usually high level enough to make it worth hurting the caster.

Its all about having the right counters.