It seems, results of battles correspond to the law of normal distribution.
The relation my_attack/enemy_defence shows a expected value of damages which will be put to the enemy unit. And on the contrary, the relation enemy_attack/my_defence show damages which can be put to my unit. The unit will be destroyed if damages is more as current HP. It is possible to receive result considerably different from expected, but as a whole, results correspond to the "bell curve".
I have noticed that one unit can destroy several units in a one turn. Pop-up help says that in case of "one unit against several units" battle, one unit attacks simultaneously all enemy units with full power. It seems, about it spoke Brit, he does not want that power grew exponential with growth number of units. And I will agree with he.
Duration of combat does not influence on damages. Your unit can appear in fire zone only at the end of a turn, but all the same will put\take full damages.
Though I can be mistaken...