When I had to fight the Pahlava in my old Luso migration campaign I would hide in cities and send armies specifically to hunt family members (the armies I fought contained large numbers of FM). I would send out spies to look for the AI moving unguarded family members around, then move my army (medium cavalry, whatever I could afford) in. Then I would just charge the left side of the FM unit since that has a reasonable chance of actually killing the man himself. You could also use assassins, but the odds never seem to be that good.