I was under the impression that the AI targetted first units that it could kill with the fewer attacks. The sovereign usually is the unit with the most hits points in your army, so it should take more hits to remove him from the battle, while units like your spearmen starts to lose attacks faster, as only a few points of damage are enough to kill one spearmen from the unit.

Seems logicall to me, but of course some situations might be better handled by focus firing on the bigger target instead of killing the weakest first. An combined reflexion between the number of attacks needed to kill the target and the average damage it can inflict might be needed for the AI, but even there it will be hard to evaluate correctly the influence of spells.