Now what you are going to need:
Weapon 1 with AT LEAST wind element (To break guards).
Weapon 2 with wind, slash or lightning element.
Both weapons needs to be different elements (but if you have both elements it should still work, if not you can equip Synergy Seal)
Weapon 1 with AT LEAST 3 Slots:
Seals:
Double Element (and/or Synergy if you have The same Element and Double Element doesn't work out for you)
Officer Assassin
Elemental Attack Up
Weapon 1 with AT LEAST 3 Slots:
Seals:
Officer Assassin
Elemental Attack Up
Attack Up+++
If you have extra slots, Equip whatever you feel is necessary but you MOST have those seals.
Enjoy killing Lu Bu with COMPLETE EASE on CHAOS difficulty.
Having Trouble? Take a look at my Video Tutorial and see my weakest character Kill Lu Bu in CHAOS difficulty with no problems: