In chapter 11, of the empire campaign, in the underground terrain, there is a wall of fire. If you pass it out you'll die. To make one unit invulnerable, make sure you have a priest with, at least, 150 mana. Then send the unit that you to make invulnerable to the fire wall and wait until she dies. But first make sure that she dies in the middle of the fire wall, or else this cheat wont work! After that, use a priest to resurrect that unit, but be careful to don't have him dead too! If the unit as appropriable placed in the middle of the fire wall, when she returns to life she will be invulnerable, but with only 1 point of life. Unfortunately, after some time, that unit will be normal again, which is, if you are in a middle of a battle, terribly dangerous, because she has only one point of life! So have much caution and make sure you have a priest around, no, very priests around! : P
P.S.: Don't ever, EVER use this cheat on Alric or other main character!!!