When you put a Shinobi in a rival's territory, the province's loyalty rating drops by a certain percentage. If this percentage is below 100%, there is a chance that rebels will appear and seize the territory (halting income and production). The lower the loyalty percentage, the higher chance of a revolt. Moreover, the higher the Shinobi's honor, the greater chance of rebellion. Every time that rebels appear in the territory, all your Shinobi in that territory gain honor. The trick is to "invade" a rival's territory will an army of five to ten Shinobi (they are very cheap, only 100 kokus). As the territory begins to revolt, all those Shinobi will gain honor. After they are sufficiently high, spread them across different territories and team them up with four to nine new Shinobi. The high honor Shinobi will ensure that revolts take place often, giving a chance for the new Shinobi to level up. Simply repeat this process ad infinitum. You can easily complete disrupt a rival's power base in very few turns, giving you the opportunity to build up armies and/or invade territories while your opponent has to destroy your rebel armies. Try targeting key provinces with production facilities and/or mining/farming territories. Note: Avoid territories with Border Forts, or else your Shinobi will die at a rate of one per turn. Also, Shinobi have no effect on Ronin territories.
It requires eight seasons to train only one Legendary Geisha. The Geisha's true purpose is to assassinate enemy Daimyo and his heirs (effectively eliminating a faction and reducing their provinces and armies to Ronin). Their advantage over Ninja is that they are not executed when they fail: They can effectively continue trying to assassinate their target forever. Increasing your Geisha's honor is important nonetheless, because it determines their chances of success. To train your Geisha, send your Geisha into a Ronin province and repeatedly assassinate the leader of any unit there. Every time you eliminate a leader, a new level zero leader will emerge to take his place, giving you ample targets for practice. A high level Geisha can easily have over a 50% chance of assassinating enemy Daimyo.
Use the following trick to make your Daimyo a Rank 6 General and 9 Honor Unit. You can easily max out your Daimyo's stats by exploiting the best feature of the Daimyo; his 10 Elite Heavy Cavalry Royal Guards are always automatically refilled at no cost at the end of the battle (with no loss of honor). This makes the Daimyo a very formidable opponent early in the game (when most clans and Ronin do not have cavalry). At the very beginning of the game, your Daimyo can even take out 60 Yari Samurai alone (assuming that they are not supported by other units). Increase your Daimyo's General Rank by fighting many battles. The more you win, the higher rank you will be. To achieve Rank 6, you will need to have 32 more wins than losses. For example, if you have 5 losses, you will need at least 37 wins to stay at Rank 6. Start and attack a nearby Ronin province which only has one unit (60 men) defending it using only your Daimyo. Your opponent will never retreat because they outnumber you. Enter manual battle mode. Do not use automatic mode, as you will always lose because of a glitch in the game. Win the battle (60 men of any class should pose no challenge to the Daimyo). After you win, the province's loyalty rating will be very low (70% or less) because the size your occupying army is only 11 (Daimyo and Royal Guards), and will likely revolt the next turn. This will allow you fight another battle against more weak units. Rebels are usually one group of 60 Yari Ashigeru or Samurai Archers. You can repeat this process with minimum risk of losing your Daimyo and achieve Rank 6 General and 9 Honor quickly. Afterwards, your Rank 6 General will boost the Honor all accompanying units in his group by 3. This will dramatically increase your battle odds and reduce your casualty ratings.