Orthus' axe won't help. The reason Mages get targeted for assassinations has more to do with the Channeling promotions (reduced chance to defend the stack, but Marksman targets the lowest-priority defender) than with their actual strength. I'm not even sure if hawks have lower priority.
They're useful for massive sight range and detect invisible to keep the assassins from coming close anyway, just have maybe 4-6 of them recon on the furthest peaks/hills they can reach every turn and you'll spot assassins miles away. Alternatively just station scouts/hunters on nearby hilltops and such as sentries. Oh and finally, if you happen to be running Aristocracy, Royal Guards can well, guard.