From a percentile standpoint, I cannot think of doing it any way besides:
For Hero - The creation of a ton of abilities to cover every possible amount of health and checking for the percentage of health to determine which ability to use.
Unit (With Static HP) - Create an ability for each unit type that you have (So, one ability for the peasant, one for a Footman, etc) and run a unit-type check and match the appropriate ability.
Either way is extremely obnoxious and time-consuming, though.