The same way you would with the players health.
You need a script running to check the health and display the proper hud for that health level.
For the enemy you would add this to their AI MAIN script.
So your state would look something like this
:state=29,healthless=99:hudunshow=hud1,hudunshow=hud2,hudshow=hud3
:state=29,healthless=66:hudunshow=hud1,hudshow=hud2,hudunshow=hud3
:state=29,healthless=33:hudshow=hud1,hudunshow=hud2,hudunshow=hud3
:state=29:state=30
Of course you would have more than 3 health levels for your bar, but this is simple for example.
You can see that each line simply checks for the health level and then displays the right one of the three huds.