Playing as Tarth I created a party of four soldiers equipped with the following:

Amulet of Assult

Band of Might

Band of the Berserker

Amulet of Life

Mace

They are listed as having a Def value of -12 (-3*4).  However, when they enter combat their defense is listed as 4,294,967,281 and they appear to be using that value as they are untouchable with either the tactical or autoresolve modes.  I created another unit without the amulet of life and the mace and got the same results.  If I recruit a single unit instead of a party of 4 of either unit the same values are listed (-3 in the card, 4,294,967,281 in the battle screen) but they can be hit as if they are using -3 for their Def value.