I don't know about boss fights, but the current implementation of AI leaders could do with a renovation. Currently they charge blindly into battle and get knocked out without fail. Adding some sort of royal guard or captains escort should improve their survivability a little, as should some basic tactical abilities. As with the total war series groups of soldiers organised into units should fight cohesively and move with tactical purpose, cav should be able to charge in(preferably into the side or behind a group) then disengage, move away and charge in again, rather than being bogged down the moment they hit an infantry group. I honestly thing that moving to a group based tactical approach would make it easier to create a tactical AI. This way separate infantry groups can be used to flank or encircle. Strong infantry can be used to hold an attack and bog down the enemy group while light infantry skirmish with thrown weapons or circle around. Spearmen can be used to hold a cav charge and protect archers while our other infantry units get busy with the enemy infantry.
Total war type control of groups of soldiers (but not using a tactical map, instead using your character to survey and control the battle field) is the way forward for the tactical game. This way the enemy leader won't just charge ahead and die, this way he'll have his royal guard, and will have some common smarts regarding retarted charges.