I would like to point out that 'obliterated to the last man' is deeply illogical for this battle. You don't outright kill all enemy soldiers, you catch them and sell them as slaves. When you have killed 30% of the enemy soldiers, they start to rout. When 60% of the enemy army has suffered casualties (by casualty, it includes people who have been routed, injured or killed), you have effectively won the battle.
Killing them to the last man only means they will fight back, you lose your own people because the enemy now has 'do or die' mindset just like the Romans against Boudicca's superior numbers. You kill soldiers to the last man for one reason only, genocide, like what the Romans did to Carthage.