So far as I can tell there is not, nor is there any way to make a unit totally invincible. You can increase their armor and defense skill, but I have never come across a value that corresponds to health. This would suggest that once the armor is defeated the soldier is dead. Being a general or an elite unit doesn't seem to improve your chances of living.
If you change the unit's stats that are known (armor, defense, attack, accuracy, etc.) then your units will be pretty well undefeatable. You will still lose a few men though.