I will not list every single officer as the majority of them you gain just by completing the story missions. I will however list the tricky ones. Another thing to keep in mind is that certain officers won't join you if you have certain officers in your group. Such as when i was trying to get lady Noh. For the life of me I couldn't get her to join and I was sure I was doing everything I needed to. I took out Nobunaga and sure enough it worked. So I suggest using the default characters from each faction just to be safe.
Jiang Wei-In shu story 2-x have his ambush succeed. This is done by luring the enemy generals to your main camp.
Ma Chao-In Samurai story 2-x make sure all the peasants escape safely and Ma Chao survives.
Cao Ren-In Samurai story 5-x save the Uesugi army in 2 minutes or less. Then escort the cavalry units to safely and finally defeat Cao Ren.
Pang De-In Shu story 3-x rescue all of the yellow generals.
Sima Yi-In Samurai story 6-x kill the 2 fleeing orochi generals thus foiling Sima's plan. They will appear after you defeat the first 2 generals on the map.
Zhang He-In Wei story 2-x meet up with Zhang and this starts a coversation. Now make sure he stays alive for the duration of the battle and your HQ is defended.
Zhen Ji-In Wei story 6-x play as Cao Pi. When the enemy sorcerers appear in the garrisons appear quickly defeat them and meet up with her. Clear the enemies from her path so that she is free to travel north. When she has gone far enough a fake Cao Pi will appear. Defeat the imposter and then finish out the mission. If not done fast enough it will not work.
Da Qiao-In Wu story 5-x have Sun Ce meet up with her before the enemy reaches her position.
Lu Xun-In Samurai story 3-x meet up with him and then escort him back to your HQ.
Da Ji-Clear every stage in the game, all basic story missions and -x missions.
Diao Chan-Complete Wei 5-x with her still alive.
Dong Zhuo-In Samurai story 4-x defeat all supply troops that are carrying the gold before they reach Dong.
Lu Bu-To get Lu you will need to impress him. To do this you have to quickly defeat him in Wu, Wei, Shu and Samurai Stories 8-x. You will know that you did it in time if a message shows up stating that Lu Bu is impressed with your achievements.
Orochi-Gain every officer other than Orochi.
Zuo Ci-Finish the story for each of the factions.
Masamune Date-In Shu story 5-x follow all given directions.
Goemon Ishikawa-In Shu story 4-x break all 10 of his treasure boxes before you defeat him. They are scattered along your path in a fairly linear path and are hard to miss.
Kunoichi-In Wu story 4-x escort all the peasants to safety.
Keiji Maeda-In Wu story 6-x you will be asked to hit a drum every so often to summon reinforcements. Once all the back up has arrived you will need to ensure that they reach Keiji's position safely before the timer gets down to 10 minutes. Then defeat him.
No-In Wu story 3-x defeat all the messengers before they can escape. They spawn from the house at the bottom of the map.
Kotaro Fuma-In Wu Story 2-x seize the supply garrisons then escort the supply troops back to your HQ. Once they are all safe defeat Fuma.
Tadakatsu Honda-You must impress Honda. To do this you will need to impress him in the 7-x stages for each faction.
Shu-Make sure all peasants reach safety.
Wei-Once the level begins quickly head south and rescue Honda's son and securing his garrison.
Wu-Kill all of Honda's general's. Do this before you fight him. Kill Honda and then kill him again when he respawns.
Samurai-Escort the messenger to safety, Move to Lu Bu's position to initiate a cutscene and after ward's Honda will appear, now make sure a Honda and Bu meet up and another cutscene will play, now kill Lu Bu, Stay away from Date and wait for your reinforcements to arrive, after they arrive kill Date. You must also ensure that all your officers remain alive.
You will know if he is impressed when a message pops up (usually at the end of the level) stating that he is impressed with your achievements.
Yoshimoto Imagawa-In Wei story 4-x defeat all of the enemy engineers before they enter the village. Now kill Shingen and Kenshin and he will appear. Begin fighting him but don't kill him. Slowly work him down and a converstion will start and he will power up. Make sure all converstions are done before you take him down. I believe there were 2 of them.
Musashi Miyamoto-In Shu story 6-x you have to guide him to the damn, and then kill Lu Bu. Also, all allied officers must remain alive.
Nene-In Wei story 3-x when the 2 clone Nenes show up defeat them within 4 minutes.
There ya go you should now have all that you need to finish off the game. Hope this helps.