It's soo nice playing as Bannor, all the benefits of Guardsman, none of the drawbacks (since all units have it). Otherwise, I've found that simply sending an advance stack of Recon units and Assasins will take the hit from most of the AI Assasins, especially since you can usually stay in cover with 2 moves. Your own Assasins (if they survive) can kill the injured AI units leaving your main stack to move in relativly unharrased by the AI.
This works great if you have Hawks and the Raiders trait, since you can find the Assasins before they even hit you and get them first. If you have a Hero, get him Guardsman and that should also stop the attacks. The AI would be afraid to attack the hero at all, and won't send Assasins at him either. Even if the Hero does get him by an Assassin or two, World Units have about a -50% strength modifier when picking the best defended, which counters the defend until you die aspect of Guardsman.