None of these will reliably work becuase when a unit is killed the AI of the unit is completely disbaled, even if you bring it back to life.
A script that constantly sets a units damage to 0, won't save anyone from a bullet to the head.
A script such as knut erics's is the best way to make units really tough...not indestructable but tough.
I'd make the script like this:
#Alive
_object setdammage 0
~0.01
?!alive _object :exit
goto "alive"