AI limb to attack logic

The Question:
How AI-controlled enemies should decide what part of body to attack?

Background:
Our team is planning on creating game where limbs can be completely cut off and require immediate medical attention.
The game is 2D top-down melee-heavy shooter. Player is human. Enemies are melee only, also have human bodies. Player can wear different gear that will lose effect after limb is cut off. And sometimes player has medieval-like shield(big or small) involved as a matter of protection.
We haven't decided if enemies are dumb or smart, but if they were smart, they would attack the weakest limb again and again, but physics shouldn't always allow this.

 

have them always aim for the torso, but make them often hit limbs instead depending on player pose?