This Guide consists of different Ways of efficiently increasing the Max Durability of your Weapon - up to a maximum of 2000 without the use of repair fairys.
For Physical Weapon users,
(i.e. Sword/Odachi/Axe/Spear/Bow/Twin Swords)
the quickest way (If they use their Main Weapons) is probably either Kishinden (Onigashima) or Battlefield (Final Dungeon of Sekigahara) - May chance with new content, dependant on where your Partners stand - since both got Yoshitsune in front of the Dungeon. (The Information about Kishinden i got from Draculina, thank you very much.)
You just go in the Dungeon and atk first mob (Either will skill or normal atk) - if you want Max the durability of multiple weapons or level up skills, feel free to swap weapon/skill after every successfull hit. After you did it, press esc and leave the dungeon. Talk to Yoshitsune and repair (You dont need to add a Repair fairy, as the chance for Bad repair is almost 0 if you havent lost much durability.)
From grinding my Spear 1200 Durability to 2000 Durability - i did it in more then just one go - split the time - average 4-7h needed - i needed 4 hours. Dracu needed 7 hours.
Another Way would be, if you plan on using Skill Cards or wand to smelt afterwards - to use Way of the Spear (Or any other Self-buff) and repair after every use of it. Since Self-buffs always lower your durability. You Probably dont have such a Skill on a Main Weapon.
For Magical Weapon Users,
(i.e. Wand/Staff)
its much easier to level up the Durability, depending on your clicking speed.
The Quickest Way would be to spamm your high-cost Spells till you dropped to almost 0 SP.
After that, the next step is (You can even do that while using the last Spell) hold Shift and click Yoshitsune - while you talk with Yoshitsune - the Shift (SP Rec/Dura loss) keeps being active till you cancel the conversation.
You Repair - close the repair window, but still stay in Yoshitsune's Conversation, so open Repair again and repeat it a last time. With Around 900 SP, you can repair 3 times before your sp is full again. So close the Conversation and repeat the First step Again.
Short : Spamm Skills till 0 SP -> Hold shift while talking to Yoshitsune -> Repair 3 times -> Close Conversation.
I first used the Physical formular while grinding a Spear to 2000 Durability, so my wand was alrdy at 1540 Durability.
From 1540 Durability up to 2000 Durability it took me around 35 Minutes.. Hands down, that is the quickest way for it.
Ofc with the Magical Varation of Max Durability Increasing it costs alot more gold. So you should have at least around 1-2m gold before attempting it from 1200 Durability. Also.. to note, if you click to slow with this strategy it IS possible to get a bad repair, if the repair cost is above 2000 gold on an attempt. But it only happened Once to me, so no draw back.
Note :
Also as another Thread already might have mentioned, you can just use repair fairy's if you want to be on the safe side, since it only uses up if you actually got a bad repair.. which only happened once to me, while farming Durability.