Dynasty Warriors 6 Cheats:

Unlock Lu Bu

To unlock the character Lu Bu, complete Mussuo Mode with the following characters: Liu Bei, Cao Cao, Sun Jian, and Diao Chan

Unlockable Difficulties

Completing Musou mode will unlock Expert
Completing Musou mode with at least one officer from all factions (Wei, Wu, Shu, and Unaffiliated/Other) will unlock Chaos
difficulty.