basically:
unit resistance, hero's resistance skill, hero's special ability (Togrim, and in the same sequence as the unique units, meaning with level-based resistance + 5), artifacts, unicorn aura.
if any element of this chain is missing, it won't interrupt the chain, but will continue without taking that element into account!
it would be simpler to just sum everything up, but the developers created this system with a certain percentage probability for the spell. if it were about the sum, Rampart would be the undisputed leader with the best hero in Heroes of Might and Magic, guess which one...
regarding the aura – put all the dwarves around the unicorn! that's what the aura is. and those people who think the aura is around the rear... I feel sorry for those people