I just started designing an strategy game where player's armies consist of stacks of units. For my design I was aiming to reproduce the following behaviour for a full combat of multiple turns (for simplicity assume both stacks are the same unit):

* 1 vs full stack: the single unit is killed, the full stack hardly notices.
* half stack vs full stack: the half stack is killed, the full stack sustain noticeable damage.
* almost full stack vs full stack: full stack wins but gets almost wiped out. If randomness is involved, any of both may win, although the bigger one has statistical advantage.

My first approach was to add up the stats and treat each stack as a single unit, resolve the combat and then calculate how many units are killed.

That didn't produced the expected results, as even the slight numerical advantage in one side produced it to win by a huge margin.

The reason is that after each turn, the bigger stack would kill more enemies than the smaller, widening the power difference for the next turn. This different increases exponentially until the smaller stack is quickly obliterated.

As a second approach I treated each unit in a stack individually for the combat (something that I wanted to avoid), which produces much better results. However now I have many choices about how to resolve the combat.

Should it be a series of 1 vs 1 or should I allow units to gang up on others?

Should I pair each evenly or randomly?

Should I leave extra units unpaired as "reserve", or all units should fight at least once?