There are actually 2 ways to acquire this achievement.
1. Hard, but free - As the other guide has explained, you must go through the Tower and fulfill various requirements. If doing this, you pretty much have to be maxed level, which in this case is 50. You should also have the 4th weapon for your person.
I suggest using Kenshin, as he has many helpful moves:
X X X X Y - Kenshin smashes the ground, sending troops flying into the air. Works well against Defense Guards.
X X X X X Y - Kenshin rushes forward. This is probably his best move. If used against a General (generic or not) one of two things will happen. A, he will stagger, allowing you to get off some hits, or B, he will become dazed. This move is your best friend.
X X X X X X Y - Kenshin grabs the enemy and throws them. This is also unblockable, they will be grabbed no matter what (unless they're in the air).
X X X X X X X Y - Kenshin shoots orbs forward, smashing anyone it the way. Does very good damage. If found that it's pretty much a 50/50 chance of breaking a Generals guard, best to use Kenshins rush attack first.
It's hard to finish the Tower, I'm at level 67 and only got up to floor 68....
2. Expensive, but super easy - Simply buy the DLC for $29.98. Go to the shop and buy the guard. You get Kojiro as a playable character, so just buy Yoshiteru Ashikaga, and save. Achievement unlocked.