Complete the level "The Battle of Hulaoguan Pass" in Part 4, and defeat the area boss Lu Bu.
Some tips for the boss fight:
Be defensive while Lu Bu is on the horse. Defelct every attack you can and wear his spirit down. When his spirit bar is maxed out, he will be knocked off the horse, making melee combat easier.
Equip a Water wizardry spell that targets (such as Frost Lance). When he is off his horse, he equips Fire magic on his weapon. Hitting him with the Water spell will cancel his Fire magic.
Hope this helps, good luck!