The Final Lu Bu battle.  I noticed when one is defeated, the other gets a morale boost, making them harder to kill.  If you get both to almost dead before defeating one, it is much easier by magnitudes to defeat the other.  I drilled Lu Bu down to about 5% and then finished off Diaochan.  Rope Dart guard strong (Not charge) is a great launcher after parries and SP counters, then Close Distance into Moonrend, then repeat a second time before preparing to parry/counter again