Unbelievable Weapon Farming Trick In Warriors OrochI 3! Xu Shu Reveals It All...

You don't need a super high leveled character. Here are the stats of my Xu Shu:
Promoted 2 times for 6 slots:
Lion's Fang
Charm Of Alacrity
Phoenix Wing
Chimera Shell
Dragon's Spleen
Mystic Bells
Weapon:
Mystic Clouds (55+36) - it is not +81, so it can be done even faster
Bastion (for add. armor)
Aggression (for damage)
Absorption +10 and Osmosis +10 (to have a way to recharge Heath and Musou gauge fast)
The rest is not super important, but it can be optimized even further.
Get speed and attack to max. Best case scenario is around 4 minutes per run and you will get 4 mystic weapons.
Also, this guide is for farming 145 mystic weapons for the Unlimited Power achievement, not getting maxed out weapons.
Enjoy!