Dear Aiman
This could work because of the variation of damage values of each unit, however I can see the problems your talking about (Example: 1 on 1 battles of the same unit would reach a stalemate) but people can buy different variations of multiple units that can kill faster than the auto heal can repair.
So there are two possibilities I thought of to solve this problem: A)Make "Magic Mitosis" a very weak auto heal (even level 3) or make it very expensive
B)Get rid of all healing "Specialties" (Garrison is the only way to heal)
Or if you have a solution please post :)
I need Ideas XD