See “Ranmaru Mori Unlocked” for preparation tips. First you need to reach floor 76 in survival mode, which is a task in itself(it doesn't matter what missions you take to get here). On floor 76, choose the mission Employment Opportunity. On floor 76, get 100 kills in 5 minutes. On floor 77, defeat the 3 officers in 5 minutes. On floor 78, again you will have to defeat the 3 officers in 5 minutes. On floor 79, you must defeat Kojiro Sasaki in 5 minutes. On floor 80, defeat Musashi Miyamoto in 5 minutes. You will have to fight Kojiro Sasaki again to clear the floor. He will now be in the shop to hire. To actually get the achievement, you have to hire him and save your game.
Other gameplay tips:
Buy a horse, this will be very useful as you often need to cross the maps quickly. If you are playing two player co-op, then you get more gold for finishing the missions. If you are having trouble with a mission where you need to protect all of your officers(or any mission for that matter), try raising the morale of your army. You get a boost to morale for completing the objectives you get during the mission and for every 50 kills. Seal any enemy gates you come across to prevent more enemies from spawning.