First make sure you have purchased the Swordsman DLC. Then start a new local game, and add two local players in addition to yourself(so you control all three). Set all three to being the Swordsman.
Each of the Swordsmen will start with a sword. Move all three characters around, always trying to get the two extra players to encounter the player 1 character in battle. Both must have a weapon at the time of combat, and they must both equip their weapons for that battle.
It doesn't matter whom is attacking or defending, but only the player 1 character should click the "Disarm" button(below the roll dice button in combat). When the "Disarm" button is clicked, the opponents weapon card will display, and you can click to roll the die. On a 5 or 6 you will disarm the opponent, and their weapon will drop to the ground. (Meaning they can't use it for this battle, and they'll have to pick up the weapon or another one before trying again.)
This should result in giving you one point towards the seven needed for unlocking this achievement. Just rinse and repeat. You might have to do this across a couple of games, but it really shouldn't take more than 20 minutes at most.