You can get this achievement without finding the rifleman 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 rifleman (costs 10 leadership points and has a rifle obviously) summon him and just keep summoning archers until the achievement unlocks. If your archers are dying, just summon a few farmers or the equivalent.
The achievement will unlock mid-wave as soon as 25 archers are holding rifles.