Wait until one of your faction's castles is besieged by the enemy then go to that castle and wait inside. After awhile, the enemy faction will attack and you will be prompted to join in the defense. Win that fight and the achievement should be yours. You can also join a battle in progress from outside of the castle but that requires fortunate timing.
I've personally found that melee soldiers are more useful than archers since your own archers get killed too easily by the enemy archers. I also go to the options and increase the battle size to 100. This seems to help your defenders in long, drawn out battles since it decreases the number of waves (and thus the number of times enemy archers will spawn and shoot you and your defenders). My first defense was against 400 to 600 attackers and it seemed hopeless for my small army until I did this.