Achievements
All the achievements are secret, but they are all relatively simple. Get the majority of the achievements by unlocking new officers (just complete mission targets on Easy or Normal) and by completing the Musou mode with each officer (on any difficulty).
Additionally, there are achievements for completing all the mission targets and clearing all the stages.
Clear Musou mode with all the available officers (solo or co-op doesn't matter, but all the officers available in Musou mode need to clear it). Additionally, all officers need to attain level 50 (which effectively maxes out their skill trees).
Lastly, take first place in the records for all challenges (in Challenge Mode): Rampage, Sudden Death, Speed Run, Havoc, and Gauntlet.

Extra Height
[Submitted by: Cheat Masters ]
Description: Some officers can get a little bit more height from their jumps.
How to: Use a Strong attack while in the air to get a little more height. With practice, you can get most officers past some high fences the horse can't clear. Since you can always summon a horse from anywhere (it teleports), you can use this to get across certain maps.

Voice Gallery
An officer who reaches level 50 (the max level) will unlock their voice gallery.

Achievements
All the achievements are secret, but they are all pretty simple.
1) Get the majority of the achievements by unlocking new officers (just complete mission targets on Easy or Normal) and by completing the Musou mode with each officer (on any difficulty).
2) There are achievements for completing all the mission targets and clearing all the stages (the latter probably pops up before the former).
3) Clear Musou mode with all the available officers (solo or co-op doesn't matter, but all the officers available in Musou mode need to clear it).
4) All officers need to attain level 50 (which effectively maxxes out their skill tree).
5) Clear all stages (not targets) on Chaos difficulty. Wishing you good luck on this one!

Unlocking New Officers :. See Those Targets?
Description: Unlock the entire officer roster
How to unlock: Check your mission targets. They are the key to unlocking new characters!

Characters
Description: The following characters can be unlocked via the techniques listed.

Cao Cao	Complete Musou mode with three Wei characters
Cao Pi	Defeat Cao Pi in less than five minutes on the Wu side of He Fei
Cao Ren	Complete Battle Objective 2 on the Lu Bu side of Fan Castle
Diao Chan	Complete Musou mode with one character from each kingdom
Dong Zhuo	Complete Musou mode as Lu Bu
Gan Ning	Complete Musou mode as a Wu character
Guan Ping	Complete Battle Objective 2 on the Shu side of Fan Castle and finish the battle with Guan Ping surviving
Huang Gai	Complete Battle Objective 3 on the Wu side of The Battle of Chi Bi
Huang Zhong	Complete Battle Objective 1 on the Wei side of Ding Jun Mountain
Ling Tong	Complete Battle Objective 1 on the Wei side of the Battle of Shi Ting
Liu Bei	Complete Musou mode with three Shu characters
Lu Bu	Complete Musou mode with Liu Bei, Cao Cao, and Sun Jian
Lu Meng	Complete Battle Objective 2 on the Wei side of He Fei
Ma Chao	Complete Battle Objective 2 on the Wei side of Han Zhong Attack Defense Battle
Pang Tong	Complete Battle Objective 3 at Cheng Du battle and finish the battle with Pang Tong surviving
Sun Ce	Complete Battle Objective 1 on the Sun Jian side of the Battle of Xia Pi
Sun Jian	Complete Musou mode with three Wu characters
Sun Quan	Complete Battle Objective 1 on the Lu Bu side of The Battle of Chi Bi
Taishi Ci	Defeat Taishi Ci in the Battle For Wu Territory
Wei Yan	Complete Battle Objective 2 on the Shu side of Wu Zhang Plains and finish the battle with Wei Yan surviving
Xiahou Yuan	Complete Battle Objective 1 on the Shu side of Ding Jun Mountain
Xiao Quio	Complete Battle Objective 3 on the Wu side of the Battle of Shi Ting
Xu Huang	Complete Battle Objective 2 on the Wei or Wu side of Fan Castle
Xu Zhu	On He Fei on the Wu side, personally kill Cao Ren and Xu Chu
Yue Ying	Complete Battle Objective 1 on the Wei side of the Battle of Wu Zhang Plains
Zhang Jiao	Defeat Zhang Jiao in the Yellow Turban Rebellion
Zhang Liao	Complete Musou mode as a Wei character
Zhen Ji	Complete Battle Obective 1 on Xu Du Invasion Battle, Lu Bu scenario
Zheng He	Complete Battle Objective 2 on the Shu side of Han Zhong Attack Defense Battle
Zhou Tai	Complete Battle Objective 3 on the Shu side of Yi Ling
Zhuge Liang	Complete Musou mode with a Shu character

Unlockable Difficulties
Clearing Musou mode once with any officer will unlock Expert.
Clearing Musou mode with at least one officer from all factions (Wei, Wu, Shu, and Unaffiliated/Other) will unlock the ever lovable Chaos difficulty.

Second Costume
An officer unlocks their second costume upon reaching level 25.

Movie Player
Clearing Musou mode with one of the 17 officers who are available for that mode will unlock the CG movie player.

More Experience From Officers
[Submitted by: Cheat Masters ]
If you want more experience when killing various officers, use your grapple attacks. Doing this will net you more experience (sometimes twice as much) than you would normally get.