This trick will take quite a long time, but is worth it. Before facing Lu Bu, charge up your Musou. Then, charge towards him (from behind, if possible). Use your Musou on him. After he is on the ground, use this chance to run away. Charge up your Musou again and return to attack him. Your Musou could be more powerful when your life bar is red. This will set Lu Bu on fire.
Take Lu Bu's Red Hare and run back and forth. Run him over and attack with Triangle to get weapon experience or just a normal Square attack. When he is almost dead, you can get off the horse and use Musou, or keep riding horse and lunge at him.
Use the following trick to defeat Lu Bu in a duel. When in Hu Lao Gate, Yuan Shao will say "Protect the ram while it heads for Hu Lao Gate". After the ram destroys Hu Lao's Gate, Yuan Shao will say "All forces march. Take Dong Zhuo's head." After that, Lu Bu will appear. A few seconds, later Lu Bu will say "You dare face the mighty Lu Bu" . Duel him. Use the following items as Zhao Yun on the easy difficulty settin to defeat him. Note: Also use Zhao Yun's Max combo, which is Square(2), Triangle(3). If done correctly, he should fly in the air, allowing you to land another combo. The items are: Tiger Amulet: 15, Tortoise Armulet: 19, Peacock Urn: 15, Dragon Armulet: 15, Wind Scroll: none. Make sure your character has almost maximum stats.
Use the following trick to defeat Lu Bu in a duel. This can be somewhat difficult to actually do, but if it works you can defeat Lu Bu in a duel easily. First, get your character's weapon to at least level 4. Going to all three of the Yellow Turban levels twice (and defeating most, if not all Generals and sub-Generals) should take care of this. Attack Lu Bu with the third charge (Square(2), Triangle) and keep pressing Triangle. You will notice that since the character has the second form of the weapon, they do more hits than before. When Lu Bu becomes dizzy from the last hit, get behind him and do a very strong combo (varies for each character). Keep doing this until he is defeated. Note: Some characters tend to push Lu Bu to the side, thus missing the blow that actually makes him dizzy (for example, Liu Bei, Sun Quan, Guan Yu, etc.). However, this does not make it impossible for them to pull this off.
To defeat Lu Bu in under ten minutes, when the battle of Hu Lao Gate starts, quickly get your gate open by defeating the General that blocks it (Li Ru, Dong Min, etc.). The battering ram should now appear. Kill the General that blocks the Shu army. After killing him, the Shu army should protect the ram. Do not kill the Guard Captains. You will need their health power-ups when fighting Lu Bu. When the gate breaks, quickly hit Lu Bu off his horse. He should now be in rage and pursue you. Do not accept his duels. When fighting Lu Bu non-duel style, just block (L1) all of his attacks. If the light shines, jump and press Triangle for a jump charge. He should now be flying in mid-air. Quickly slash twice before he guards and counter. After jump charging, guard again. It is best for Lu Bu to let him decrease your health to red so you can Musou rather than slashing. Make sure health is nearby. When his life gets low, his Musou attack will shatter your defenses if not blocked correctly. Make sure you are in a stance in front of him. If he Musous, execute your Musou as well if you want to do damage. A stalemate will start; press Square rapidly.
First, get your character to level 8 or lower. Then, get your weapon to level 5 or higher. Finally, level up your bodyguards until you get 8. At the beginning of the battle of Hu Lao Gate, defeat at least four officers. When the ram appears, do not kill the guard captains. Wait until the ram breaks the gate and Lu Bu appears. Knock him off the Red Hare and do not challenge him to a duel. Get him to chase you, and run into only one other General. Get that General to attack him. Keep your distance and try to use your Charge Bow to knock him out (hold R1, then press Triangle). When he is knocked out, go up to him and perform a combo. If your life is red, keep your distance and let the General and your bodyguards attack him. When you get your Musou, use it on Lu Bu. Keep doing this until he dies. You can kill a Guard Captain to heal.