When you want to have some more realism, and/or don't want to spend effort animating many kinds of different death/dying animations, rigidbodies and physics come to the rescue. Unity has, since the earliest versions, a handy tool that allows you to specify the rigged character skeleton bones into ragdoll slots and then, with a single click, to create all the components necessary to have a realistic ragdoll simulation for that character.
We will use the harder approach this time, which will be not to have a copy of the guard without components with just ragdoll colliders and rigidbodies, and instantiating it, when needed, in the place of the guard. In this game we are not killing the guards by design, instead we are just temporarily stunning them, so, they will soon be back on their feet. There is no reason to write a special class to no point to use that we will write a special class to take care of enabling/disabling those components at runtime. First things first, we will create the ragdoll for the guard prefab.
Select one of the AI guards in the hierarchy, then open the main menu and select GameObject->3D->Ragdoll. This will bring up the Unity Ragdoll creation wizard.
As you can see from the wizard, it will be required that you drag the guard skeleton's corresponding bones to the wizard slots. For example, we will drag the LeftUpLeg child in the Left Hips slot, and we will drag the Hips child in the Pelvis.
Leave 20 for Total Mass parameter and 0 for Strength:
After pressing Create, you will see the guard GameObject filled up with colliders, rigidbodies, and phsyics joint that link rigidbodies. Test the ragdoll by pressing the Play button and observe the simulation.