I’m less interested in talking about what class/stats Lü Bu would be as a Fire Emblem unit and more interested in his story; it’s a great example of the Three Kingdoms mythos having a lot that Fire Emblem stories can learn from. Even though Lü Bu was considered by many to be the most powerful warrior in the world, his competence on the battlefield didn’t translate to diplomacy well at all. He did a lot of waffling on whether to betray Liu Bei and/or make an ally out of Yuan Shu (yes, Yuan Shu is the last person you want as your only ally, but he’s better than nothing) and his irrational actions ended up screwing him over when Liu Bei was willing to join forces with Cao Cao, his rival, to take Lü Bu out.
There is the Dynasty Warriors narrative of Lü Bu going down fighting and dying with pride instead of surrendering, but the reality is that he’d already made his own bed by burning all of his bridges before the last battle even happened. It’s a great example of cause and effect at play in narratives: you’ve got this amazingly powerful guy who seems invincible, but then when he gets defeated you can look back and see the circumstances that lead to his undoing. Reminds me of Sigurd in FE4: his fate feels like a surprise twist at first, but if you look back you can see him walking into it every step of the way.
As for who could beat him? FE1 Marth with the Falchion because the FE1 Falchion makes it impossible for enemies to attack you. The best circumstances Lü Bu would have against him would be if he were on a throne/gate that restored his health faster than Marth could damage him and they would be in a stalemate… Unless Lü could make some kind of Faustian deal with Medeus to get past the Falchion’s protection. Man, I wish FE Warriors had been a Dynasty Warriors crossover. That would’ve been rad.