Got a few tips that really helped me:
1) if youve got coastal cities, have atleast 1 inexpensive ship watching the water each direction. The AI is known to make naval invasions; a heads-up a few turns in advance of a coming invasion force is crucial. If you have the money, have one big fleet ready, since killing a navy kills the army embarked on it and the ai rarely has a full stack.
2) have your agents be more proactive. Even failing grants experience (unless they die). You should have atleast 1 agent specialized in killing/arresting/converting other agents. If you got a fresh agent, have them do easy tasks to get those lvls necessary for getting rid of enemy agents. Its rock paper scissors for agents of equal lvl, if your agents' got a large level advantage, it wont matter who they fight
3) If your enemy's spamming one type of agent, counter it by investing heavily in its rival. The circle of what beats what is: Monks/priests < ninja < metsuke < back to priests/monks
4) trade is only suppose to get your economy kickstarted. Invest that money in farms, roads and markets. All of these provide growth (farms indirectly since a food surplus gives growth) Growth is important for when you eventually have to become self sufficient
5) dont upgrade your market past rice exchange - the commerce increase doesnt really out weigh the growth penalty (consumes too much food)
6) if youve got a raiding problem and you cant afford to fix it at the moment, dont. Buildings get repaired over time at not cost, youll just miss out on what ever that building produced. Just garrison an army at your raided town. Eventually the enemy army will have nothing to raid and will either retreat, attack, or get past you. The first two are preferable.
7) army composition- make sure you have a yari ashigaru core army(especially as oda since they have the best). Hire samurai units sparingly, theyre mostly just to flank or protect the flanks. Yari ashigaru in spearwall is amazingly cost effective from the front. Im talking katana samurai losing 80%of their unit before killing 60% of the spear wall. The only thing that really beats them head on is heros and an enemy yari wall, but neither are really common. Have atleast one samurai archer unit mixed with a few ashigaru archers. Samurai archers do most of the killing but the ashigaru archers soak up enemy missiles. Up to you how much you want cav, personally i only have 1-3 in an army. Having an ashigaru core army keeps it cheap but effective.
Good luck, I'll add more if I think of any