i think i see the problem. i would suggest thinking of this as a real situation.
say you have two groups of soldiers, and you want to guarantee that one side always triumphs, and the other side always retreats. how would that happen in real life?
if the losing side were all novices and the winning side were experts - set the skill level of your loons accordingly.
if the losing side ran out of ammo. again, set the ammo sliders.
if the losing side had more wounded, and so weren't able to aim as accurately. set the health sliders.
if the losing team were nervous cowards and had a tendency for insubordinate running away; there's a command called allowFleeing. make it 1 for the side you want to run away, 0 for the winners.
if the winning side had a bunch of reinforcements running up the hill to join them.
all this is pretty standard stuff though, and to be honest, i'd suggest simply waypointing it to happen as you want it to, just as gubes suggested, using the condition box to set up at what casualty number the losers start running.
hope this is of use