If you second the unit system, I suggest you to not think in terms of turns like berserkers attack archers, then archers attack bersekers, or you'll end up with a dynamic in which the first that strikes wins (and since it is realistic in a duel, it is not in an army clash).

You can solve the issue in two ways.

1) Among the confrontation of bonuses, you add a 1d10 based table. In a drawish situation 1 means the attacker loses %40 of troops (to say) and the attacker loses %0; 5 means both attacker and defender loses 20% of troops; and 10 is the reverse of 1. This way you no longer need to calculate hp, damages and so.

In this way, you should consider that individuals in the unit are like hp and, given a loss of X, the unit is disbanded.

2) When the two armies confront, they deal each other damage to the hp total (in your example 320). Any unit doesn't report casualties until it had at least a 20% hp loss. To 20% and until 50% loss, the unit loses a percentage of troops equal to the hp loss percentage -20% (ie, after a 50% hp loss, the unit has a 30% troops loss). After the 50% and till 80% of hp loss, the troop loss is at -10%. After it is on par (but you should really consider to make a unit disband at this point)