Dynasty Warriors: Strikeforce

This one took a decent chunk of time. The requirements for the achievement to unlock are as follows:
1) All 3 stories completed through chapter 6
2) All request missions for all 6 chapters complete
3) All Legend(Duel!) quests complete
4) All Crossover quests complete(obtained from the guy between the bulletin board and the storyline quest giver)
5) All Gaiden quests complete
6) The 3 Nanman campaign quests(Given by the officer in town in chapter 3 ONLY)

The best advice I have for this one is to put it on easy to minimize the time it will take, also the High Difficulty versions of the story quests have no impact on your quest completion %, so be thankful for that.

Here is a complete list of all missions needed for this achievement. I just finished the game and typed out all the missions that I could find that went towards this achievement. Other guides will give better details on some of the missions, but I thought a list would be nice to keep count and cross off ones that you are taking care of, especially the Gaiden missions which I was unsure if they would ever end! Like the other guide says, the High Difficulty missions are not needed and keep an active eye on the people in your kingdom such as the Nanman guy in Ch3 and the characters who give you Crossover Missions. The total numbers that I have provided include playing the missions for each story. For instance, in CH1 there are only 7 missions but you have to play them 3 times for all 3 forces.
Please contact me or comment if there are any issues with this guide, especially if I left any missions out or miscounted or something. I hope this helps and will make the missions seem more manageable!

Story
Ch1 (21 total)
For Wei, Wu, and Shu
Battle Quests: Yellow Turban Menace, Yellow Turban Fortress, Yellow Turban Rebellion
Requests: Bandit Trap, Officer Pursuit, Lost Belongings, Surprise Attack

Ch2 (24 total)
For Wei, Wu, and Shu
Battle Quests: Battle of Si Shui Gate, Battle of Hu Lao Gate, Battle of Luo Yang, Battle of Mei Castle
Requests: Liberation, Turbans’ Remnants, To the Rescue, Ambush

Ch3 (27 total)
For Wei, Wu, and Shu
Battle Quests: Pursuit of Lu Bu, Battle of Chen Liu, Battle of Xia Pi, Battle of Bai Men Tower
Requests: Breaking Through, On the Hunt, Rations Defense, Swift Tactics
Nan Zhong Requests: Invasion of Nan Zhong, Nan Zhong Struggle, Unification of Nan Zhong

Ch4 (33 total)
Wei Specific
Battle Quests: Escape from Wan Castle, Battle of Guan Du, Battle of Bo Wan Po, Battle of Chang Ban, Battle of Chi Bi
Wu Specific
Battle Quests: Assault on Liu Biao, Assault on Wu Territory, Unification of Jiang Dong, Battle of Xia Kou, Battle of Chi Bi
Shu Specific
Battle Quests: Battle of Bo Wan Po, Battle of Xin Ye, Battle of Chang Ban, Battle of Chi Bi, Assault on Cheng Du
For Wei, Wu, and Shu
Requests: Captivity, Age of Chaos, Military Funds, Two Front Defense
Wei Specific
Requests: Guan Yu’s Escape, Cao Cao’s Siege
Wu Specific
Requests: Golden Seal, Merging Forces
Shu Specific
Requests: A Duo Rescue, Three Visits

Ch5 (36 total)
Wei Specific
Battle Quests: Battle of Shi Ting, Battle of He Fei, Battle of Jian Ye, Battle of Mt. Ding Jun, Battle of Wu Zhang Plains, Battle of Cheng Du
Wu Specific
Battle Quests: Battle of Fan Castle, Battle of Yi Ling, Battle of Cheng Du, Battle of Shi Ting, Battle of He Fei, Battle of Xu Chang
Shu Specific
Battle Quests: Battle of Mt. Ding Jun, Battle of Wu Xhang Plains, Battle of Xu Chang, Battle of Fan Castle, Battle of Yi Ling, Battle of Jian Ye
For Wei, Wu, and Shu
Requests: Bandit Hideout, Steel and Fire, Path of Destiny, Martial Trial,
Wei Specific
Requests: Run the Gauntlet, Iron Armor
Wu Specific
Requests: Strategist Gathering, Two Paths
Shu Specific
Requests: Ma Su Rescue, Blood of a Warrior

Ch 6 (24 total)
For Wei, Wu, and Shu
Battle Quests: Sealing Death Phoenix, Sealing Death Tiger, Sealing Death Dragon, From the Depths
Requests: Black Storm, Bewitching Land, Manipulation, Zenith Tower

Crossover (13 total)
Orochi’s Ambition, Orochi X Resurrection, Kiyomori Taira’s Fury, Da Ji’s Schemes, Fast and Furious, Femmes Fatales, Sorcery Struggle, Bandit Infiltration, Outpost Capture, Foil the Plot, Nuwa in Need, Purge the Unclean, Mystic Meltdown

Gaiden (52 total)
Ice Bi Xie appears!, Ice Tian Lu appears!, Dragon Assault, Phoenix Assault, Tiger Assault, Bird Chaos!, Jade Trial, Wei Arena, Wu Arena, Shu Arena, Return to the Arena, Warrior’s Arena, Strategist’s Arena, Battle of He Fei X, Battle of Wu Zhan Plains X, Battle of Fan Castle X, Battle of Shi Ting X, Battle of Chang Ban X, Battle of Yi Ling X, Escape from Wan Castle X, Unification of Jang Dong X, Assault on Cheng Du X, Eternal Matrimony, Clash of the Couples, Xiao Qiao Rescue, Enduring Freedom, Way of the Warrior, Rations Required, Beautiful Brawl, Protect the Dragon, Comrades Eternal, Rhapsody in Red, Strategist Struggle, Guan Yu’s Challenge, Ice Test, Wood Test, Earth Test, Fire Test, Metal Test, Dark Test, Light Test, The Bodyguard, Bows and Arrows, Father Figures, 5 Tiger Onslaught, Reclaim the Seal, Ruler Pursuit, Cao Cao in Trouble, Strongest in the Land, Respect one’s Elders, Warriors Wanted, Last Rites

Legends (42 total)
Duel! Xiahou Dun, Dian Wei, Sima Yi, Zhang Liao, Cao Cao, Xu Zhu, Xiahou Yuan, Xu Huang, Zhang He, Cao Ren, Cao Pi, Zhen Ji, Zhou Yu, Lu Xun, Sun Shang Xiang, Gan Ning, Sun Jian, Taishi Ci, Lu Meng, Huang Gai, Zhou Tai, Ling Tong, Sun Ce, Sung Quan, Xiao Qiao, Zhao Yun, Guan Yu, Zhang Fei, Zhuge Liang, Liu Bei, Ma Chao, Huang Zhong, Wei Yan, Guan Ping, Pang Tong, Yue Ying, Diao Chan, Lu Bu, Dong Zhuo, Yuan Shao, Zhang Jiao, Meng Huo

Total missions needed for 100% completion (272)