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 and holds a hammer) summon him and just keep summoning farmers until the achievement unlocks.
The achievement will unlock mid-wave as soon as 25 farmers are holding uber-swords.