It sounds to me like you want a two-stage trigger that first sees which unit is damaged, then second sees if any damaged units are destroyed.
Why not just check outright if a unit is destroyed? Wouldn't the result be the same?
In the condition field, put the following code:
"not (alive _x)" count [unit1,unit2,unit3,...,unitN] > 0
This code will count how many units in the array of units are not alive (are destroyed). If more than 0 (i.e., at least 1) are destroyed, the condition is met and the trigger will activate.
If you meant something else, explain what you meant indepth, and I'll try to help.