Oh, if you think fighting Lu Bu is a pain in the ass, it has nothing on recruiting him.
Okay, so. The Warriors Orochi games are basically "pick a faction and lead them to glory against everybody else"... for the first two games. The various factions are the typical 3/4 (+assorted extras) sides from Dynasty Warriors and a couple groupings from Samurai Warriors; which makes perfect sense when you remember this subseries is literally taking place in a magic world created for the purpose of bring both series' casts together. Also here are "Mystics", characters from various Chinese and Japanese mythology; some are good and help the various factions (like Nuwa and Taigong Wang), while others (like Orochi, Da Ji, and Kiyomori Taira) are... less than helpful and lead/side with the demons that serve as the bad guys.
Then we get to Warriors Orochi 3; Orochi was killed last game, and things were peaceful for a time. But now its humanity and allied Mystics vs the demons and hostile Mystics... and the emergence of the "Hydra" has taken out most of humanity, to the point that the first mission is a last ditch attempt by "the Resistance" to beat the Hydra with the help of the last three known surviving player characters.
Unsurprisingly, it fails, and the three PCs were about to meet their end when Kaguya shows up and takes them back to before the battle. From there, the overall goal is still "beat the Hydra", and the method is simple: Using Kaguya's powers of time travel, go back to previous losing battles and win them to both save people (aka other playable characters) and improve the Resistance's position for the final battle.
Most recruitments in the game require you to either beat a mission to save/recruit them... or beat a mission, see them die, do another mission to alter the setup for the first mission, and beat the altered first mission to save them from their fate.
Lu Bu... is not that easy. He simply shows up in a few missions, and is generally more focused on the demons... until Nezha (one of the new Mystics for the game) shows up. This is bad, because Lu Bu is to Nezha what we are to Lu Bu. Yeah.
So how do you recruit Lu Bu? Save his lover (wife?) once the story gives you the opportunity, put her on your team, pick a mission where Lu Bu shows up, then run up to Lu Bu as her. Catch is, this won't interrupt mission dialog, and Lu Bu is usually assigned to the "third" force in an otherwise two sided battle, and if he dies, he can't be recruited. So you have to keep him alive (against Nezha, joy) without pissing him off or failing any critical mission objectives (because those are also a thing you have to deal with at the same time).
And before you say "just skip him"... Warriors Orochi 3 has three endings. The first is simply do the final battle again. The second is the final battle, again, but this time you have dedicated anti-Hydra weapons; winning this version of the battle unlocks a new stage. Going into this second ending stage without all the characters you can recruit gets you the second ending. For the third (and best) ending, you have to recruit all the characters (minus a few unlocked by beating these final stages), beat the second ending stage, and then beat the new third ending state that unlocks... although if you're playing Warriors Orochi 3 Ultimate, you still have the Ultimate-exclusive extra story that unlocked at this point...