To get Yojimbo's Break Damage Limit, you need to get Auron's Celestial Weapon (Masamune).

If you check out the Ultimate Weapon Walkthrough in this forum here it tells you how to get it:

http://forums.eyesonff.com/showthread.php?t=22160

If you need any more help getting it, let me know. I got Auron's Celestial weapon and I thought it was one of the easiest to get. (And I suck at games, so if I can get it I'm sure you can n__n)