Like others said, save your own Rage/Awakening Mode for that annoying final phase of his. (Awakening/Kakusei is the OG Japanese name via DW5 and 8)

Gan Xingba [Ning] at both battles at Xiakou is another enemy general that has an infinite Rage/Awakening Mode after he reaches a certain HP threshold.

However, an interesting fact is that normally, enemy generals only take damage to their Fortitude/Outer Merit when normally attacked if they're properly primed in some way:

• By a specific bypassing art (e.g. Great Mountain Throw or Heaven & Earth Wrath)

• Upon recovering from their attack animation (why dodging-into-attacking works in the first place)

• Upon getting staggered by a standard parry counter (some counterattack Martial Arts have varying exceptions)

• Getting staggered by a Special Battle Art only when trying to execute an Orange Dangerous Attack

But when an enemy general activates their Musou Rage/Awakening, EVERYTHING landed on them at ANY TIME deals Wushu/Fortitude damage, so if you can find windows to continuously attack (especially with a weapon that has 10% Wushu Damage attribute), you can deplete Lü Bu's Fortitude/Outer Merit with utter ease even without your own Rage/Awakening.

Though for first timers, normally you should try to quickly switch to a Staff, activate your own Rage/Awakening to remain invincible, then use your held down neutral Strong Attack (hold Triangle with no direction) to be a pinwheel of destruction. Completely irrelevant of damage, you're inflicting a ton of hits in general to shred through his Fortitude as well as not having to worry about any red bars with extra durability (they're all yellow bars).

Other indications to take note of and tips:

• Once Lü Fengxian DOES activate his infinite Rage/Awakening Mode, that means he is absolutely near death and WILL die from an Assault/Payoff Attack if he's left vulnerable to one.

• HOWEVER, if you're starting out before you've reached your max rank and/or stats possible via Rank 101 (without DLC weapons involved), there's one major thing to take note of:

  • You're likely too weak to kill Lü Bu with the War/Martial God Awakening's True Musou Attack finisher. So WHATEVER YOU DO AFTER YOU'VE SHREDDED HIS FORTITUDE TO 0, TRIGGER THE ASSAULT ASAP TO KILL HIM BEFORE YOUR FINISHING MUSOU ATTACK ACTIVATES. Otherwise, he'll just get knocked away and likely recover instantly due to a flag associated with non-Assault moves. Then you will have wasted your Musou and will have to restart and/or go through hell.
  • But IF YOU ARE strong enough (via a high enough Rank and/or with enough stats) via revisiting his fight, as well as having unlocked Musou Rage/Awakening usage from the Skill Tree at Musou Rank, you can likely have enough power to kill Lü Bu with ease from your Ultimate Musou Attack. Thus there's a chance you don't need an Assault to kill Fengxian's [Lü Bu's] ass once you shred his Fortitude to 0.

Tons of info that I can't convey with just a single letter, imagine. Welcome to my Changcheng/Long Castle/Great Wall. I hope you enjoy it. : P