Demon Shogun is the final boss at wave 50, beat it and achievement unlocks and you will unlock "This land is safe" achievement.
My farmers are at level 10 (max), I also have swordsmith and use this combination all the time in wave 50. I eventually stays very close to the gate (level 3) and use abilities "Lethargy" - level 5 & "Troop trample" - level 1 at all times.