A good speedrun tactic is to get to the first position, getting the most assets, and then kill your leader, this only works for the first two leaders, at the Shogunate battle you will need an army bigger than all the other opponents armies combined.

To get the most assets you will need to:

(There is a max. for each thing, so don't worry about leveling up something at max. level, you can check your levels in the F1 tab after selecting and clicking Return on your profile)

• Honor: Killing ronnins, samurais that dare you, donating to the local monks, marrying and helping your leader with the clan honor.
• Fief: Increasing the taxes is an easy way to level this up, also helping your leader with your army and conquering clans at the Shogunate battle.
• Warriors: The amount of loyal warriors you have, you can get more at your state if you have enough fief.
• Generalship: The trained your warriors are, "Drill your troops" options in your estate.
• Swordmanship: The trained you are with the sword, this doesn't add a real useful thing, but it's useful to get more points to be the favorite.