Assasins

[thejeff]

That's the most straightforward way.

Other strategies are to boost PD/patrolling high enough to catch the assassins before they can do to much damage.

If you're worried about your expensive mages being killed, hire a lot of cheap scouts and indy commanders to play target.

Don't forget to script spells for your mages to defend themselves. This tends to conflict with optimal battle scripting, so I usually protect battle mages with guards and let research/etc mages protect themselves except when I know their province is being targeted.