Is it possible to defeat Lu Bu during the Battle of Hulao Gate?
Solution 1:
It is possible to kill Lu Bu in this mission. But indeed, all of his combat parameters are extraordinarily boosted. Even a max level character may have issue facing him. Story-wise, you're not meant to beat him, but in general a fun challenge throughout all of the different editions of this series (and sometimes a requirement to unlock stuff) is to defeat Lu Bu in this stage.
One example of a strategy to take him down in this version is to abuse invincibility from Musou and Rage Musou. Gather up a herd of enemy soldiers in advance, with a full Rage ready and Musou as well. Then, once you lure Lu Bu out, first use your Musou (ideally the most damaging one for your character). When that's done, engage Rage to refill your Musou, and use the Rage Musou to attack him and carry him into the enemy herd you had earlier. You should ideally do this fast enough to enter your True Rage Musou and start to really destroy him. Equip a weapon that regains musou on kills to keep the rage attack running much longer while you crash through the enemies carrying Lu Bu.
If you fail to defeat him in this and you're out of resources, don't fret! Just past the gate are more enemies! Use them to build up your Musou and/or Rage and take care not to engage Lu Bu directly unless you're invincible. You should have enough enemies in the hallway to take him down, unless your weapon is excessively weak or you're piddly level, in which case you might need more. This kind of strategy can work with most characters, but it's easier with ones like Yue Ying and Wang Yuanji who hit hard and fast with their True Rage Musous.
There are other strategies available, usually based around knowing how he attacks and not letting him catch you with his musou attacks. Ranged weapons can also be used to try and attack from a much safer distance, but no matter the strategy, caution must always be exercised since he can generally slay you in a single hit.
Solution 2:
I just found out a way to beat Lu Bu. It's a pretty cheap method but it works! :D Just keep using weapon switch as soon as he gets close to deal damage and knock him back without being hit, and keep spamming it until you defeat him