Doesn't always work. Once I tried it and fought several battles with the same units in the same terrain. Outcomes were different. Besides, in a campaign battle you will never have a pure 1 v 1 unit melee. One of the units will have more valor, less armor, more fatigue, less morale, etc. Moreover, there always will be other units close by which will also factor in. So my advice is to fight custom battles with several units (much the way you will have in a "real" battle).
Bookmarks