Select side
Win the game with the Lui Bei or Cao Cao. Select "New Game" and you will be able to choose which side to be played.

Hidden battles
After you finish the game as Liu Bei to unlock Cao Cao's side, notice that the difficulty level is set at normal instead of beginner (when you played as Liu Bei). Win the game as Cao Cao under the normal difficulty setting to unlock expert level. Select either Liu Bei or Cao Cao at the expert difficulty setting. Win the game under expert difficulty setting to unlock a hidden battle for each side. As Liu Bei, the hidden battle is The Battle of East Lau. As Cao Cao, the hidden battle is The Battle of Chang Zheng.

Get a better army!!!!
If you are defeated and retry the mission you get more army

For example on the mission you run away (lui bei) stopping lei bei and letting him die can help easily build up you army (though I think this only works a few times) and can easily get your armies over 10,000 people

Kessen 2 (PS2) hints
# Select side

Successfully complete the game with Lui Bei and Cao Cao. Select "New Game" and you will be able to choose which side to be played.

# Extra difficulty settings

Successfully complete the game with Liu Bei to the normal difficulty setting. Successfully complete the game with Cao Cao on the normal difficulty setting to unlock the expert difficulty setting.

# Hidden battles

Select either Liu Bei or Cao Cao, and successfully complete the game on the expert difficulty setting to unlock a hidden battle for each side. As Liu Bei, the hidden battle is The Battle of East Lau. As Cao Cao, the hidden battle is The Battle of Chang Zheng.

Kessen 2 (PS2) Game Shark Codes
Game Shark Codes

Master Code (Must Be On) 0E3C7DF2 1645EBB3
0C0BF9CF F88D2FBF

Liu Codes

Max Troop Strength 1C609690 F8FCFE66

Max Supply 1C609691 F8FCFE66

Max Technology 1C609692 F8FCFE66

Super Troop Strength 1C609690 F8FCFE84

Super Supply 1C609691 F8FCFE84

Super Technology 1C609692 F8FCFE84

Cao Codes

No Troop Strength 1C609694 F8FCFEFE

No Supply 1C609695 F8FCFEFE

No Technology 1C609696 F8FCFEFE

Max Troop Strength 1C609694 F8FCFE66

Max Supply 1C609695 F8FCFE66

Max Technology 1C609696 F8FCFE66