-An archer would have like 1/500 chances of "instant killing" a Paladin, where as a longbowman might be like 1/300, and a halberdier would be like 1/150 or something.
The units would still have there HP, but their HP could be completly depleated with the "instant kill". Also, certain units would have a greater chance of getting or resisting an "instant kill", for example...
-A samurai would have a greater chance of an "instant kill" against all units, where as a war elephant would be nearly immune to "instant kill"
Also, ther would be techs to reaserach that improve the chance of an "instant kill". For example...
-A tech like "weak points" would increase the percent of a succesful "instant kill" by like or something, and researching a tech like "bigger shields" would increase the resistence to an "instant kill" by like .
Also, of couse, blacksmith techs would increase the chance of resisting or performing an "instant kill".
"Fool, I have auto-dialing!" -Techno Bill