Entity ia condition

before starting, I speak French and 
this is a translation.
 so I want my mob to attack when
 it has more life than the other
 targeted mob (the procedure step, 
I know how to do).
the problem is that when I start 
in the conditions of the attack block,
 I only see the dependency of a
 single entity (surely that of the mob
 that I want to create) and not that 
of the mob that must be attacked 
( because I have to know how many 
hearts he has).
 so how do I do it?