You can get this achievement without finding the swordsmith on the battlefield in a present. Every 5 waves you will play a bonus mission where you cannot attack, you can only summon. It seems like any summons picked for you are pretty much maxed out and it is nearly impossible to lose these waves.
When you have a bonus wave with the swordsmith (costs 10 leadership points) summon him and just keep summoning farmers until the achievement unlocks.
The achievement at the end of the wave providing you do not summon any other types of units. Also, I can not confirm whether or not your abilities will void the achievement.
Might as well knock out this achievement as well: