In order to unlock this achievement, you need 100 bad repairs. Yoshitsune (purple haired girl) has 3 possible results when she fixes something. Great success/good/bad repair. Bad repair is the result we need, and although the rate at which you get bad repairs seems to stay the same as you level Yoshitsune's friendship level, you still ideally want to do this when she's as low as possible. You have a higher chance of getting a bad repair when a weapon has zero durability, and bad repairs lower durability. The higher Yoshitsune's friendship is the less durability a weapon loses when you get a bad repair, but since we want to get the weapon to zero durability as fast as possible, the more we lose with each bad repair the better. There's a few ways to do this.
The first would be 4 staves. The more self-cast skills (like first aid) the better. Just spam all the skills on your weapons until you have no SP, then hold
to charge SP. Charging SP uses up durability, and any self-cast skills do as well. Once all your weapons are 0 durability, repair them all with Yoshitsune and hope for a bad repair.
An alternative, and my preferred method, is collapsing "X" weapons. Each type of weapon has a skill whose first word is collapsing and the second word varies based on the weapon type. Spear's is collapsing blade for instance. These skills use up a shit ton of durability when used, anywhere between 100-200 per use. So, you get 4 weapons with this skill (I had a sword, a spear, and 2 bows) and then go kill stuff with them. Use the skill on one weapon, switch to the next, and cycle through until all of them are at 0, then go to Yoshitsune and repair them and pray. I did it outside of Izumo Rest Stop, as the flies/daruma/red dragon things all drop items you'll need for the purity magatama spam the 1000 magatamas made achievement requires. I didn't do this until level 79 though, so they were all one hit kills.
My sword used the most durability per use, and bows used the least durability. You're looking at about 6-7 uses to get a typical weapon to 0 durability, but each time it gets a bad repair, the durability goes down and it takes less hits. I needed 3-4 uses to get to 0 durability when I was getting closer to 100 bad repairs, thanks to all the durability loss.
As an aside, don't go for the 100 great success repairs until you've done this. Oddly enough, I got quite a few great success repairs while grinding for bad ones, and the opposite isn't true. While grinding great success repairs, I didn't get a single bad one.