I used a lord only army for this one (Alberic from Bretonnia). An easy way to do is send your 1 lord army around with another full army, then find an enemy full stack. Fight it first with your full stack and deplete (but don't wipe out) as many units as possible (autoresolve is good at this). Then finish off the remaining units with your Lord only army and you should get it. I played the battle manually and the achievement popped at the end of the battle.