Jan 1546
This scenario is set in the year of Nobunaga's coming of age.
As it is the second-earliest start date, most daimyo are still in the process of growing and have comparable strengths, notably the Miyoshi, Hōjō, Otomo, and Nagao.
Famous rulers like Oda Nobunaga, Uesugi Kenshin, Chōsokabe Motochika, Shimazu Yoshihisa, and Date Masamune are not yet in power; most heads of household are from their fathers' generation.
After a few years have passed, famous characters (Kinoshita Hideyoshi and Hidenaga, Shimotsuna Rairen, Shima Sakon, Sakakibara Yasumasa) will come of age, so it may be a good idea to enlist them by Distant Employment.
With the introduction of the PKTB - Muskets Arrive scenario, there are no longer any daimyo that are exclusive to this scenario. For those who do not own the PKTB DLC, though, this scenario remains the only way to play as:
The only "solo daimyo" (starting with only 1 officer) is the Akizuki clan.
Before the event consolidating with the Tanemune Date fires in 1548, don't act indiscriminately, but instead focus on domestic affairs - then crush the neighboring factions after the consolidation. There will be a severe shortage of officers in Tōhoku until the next generation; it may be advantageous to pick off officers from the Satake as they are being destroyed by the Hōjō, but it's not worth overextending to achieve this. Improving the Risshaku-ji landmark (which improves officer promotion) can also ease the shortage. Once your influence grows a decent amount, it may be tempting to head south and do battle with the Hōjō, but bear it for just a bit more and focus on the unification of Tōhoku first.
In the mid-game, the Nagao may be about to be crushed by the Takeda and Hōjō; managing to pick off all their officers will greatly simplify things in the future. Once Tōhoku is consolidated and the Nagao are absorbed, begin full preparations for war with the Hōjō, and the assimilation of the Takeda afterward. Activating the Nanbu's "Majesty of the Full Moon" and Namioka's "Guardian General's Flag" will allow you to expand quickly and effectively.
After defeating the Takeda, attack the Oda as soon as you've recovered - if they are allowed to conquer the Mōri, Chōsokabe, and Shimazu, they will be exceedingly difficult to deal with.
In the latter half of the game, Date Masamune will appear, which will make battles easier.
Another author's strategy (hard difficulty, wide battle scope):
Aside from your father (Tanemune), most of the surrounding daimyo only have 1 castle, meaning it's recommended to attack aggressively at the start of the game. Making good use of Sagetsusai's confusion skill in small-scale battles with equivalent forces will make it easy to expand without too much trouble. Since your base at Yonezawa will put out comparatively many more troops than the surrounding castles, let's assume they will march as a group, and assign Sagetsusai as your conservator.
As you send out your troops, Tanemune's side will attack your allied forces, but do not send out defensive reinforcements. For the sake of easier expansion later on, it's better to let them steadily attack you. Depending on Tanemune's movement, if everything goes well, by the time the civil war ends you'll have scaled to a point where you can fight the Takeda and Hōjō head-on.
After the end of the civil war, even though this completely ignores the event, I recommend retiring and making Tanemune the head of the clan; thanks to his mid-level court position, your prestige will suddenly increase to nearly 400 (unfortunately, the Oshu Tandai position is lost in doing so). While Tanemune is slightly inferior to Harumune, his traits make diplomacy easier. Harumune's unique trait is somewhat difficult to utilize as a daimyo; not having to constrict your play to maximize it is a benefit. You will no longer be restricted by a shortage of funds and can assign policies freely. While we're at it, using this "retirement loop," you can mass-produce Senior Retainers.
At the same time as the end of the civil war, your initial alliance should expire, so think about what power you'd like to ally ahead of time. I recommend extending the alliance with the Nagao, as they are useful should you fight the Ashina and Hōjō. If all goes well with your expansion, the Ashina shouldn't put up much of a fight; the Hōjō, at first glance, seem to have overwhelmingly superior numbers, but if you go to battle you almost certainly won't lose (since they have so many castles, it's likely they'll send representatives/chamberlains into battle due to a lack of castle lords). Focus on fighting centralized, major battles, and you should defeat the Hōjō without too much difficulty.
In this situation, the unification of Tōhoku may be delayed a bit, but at this point it should be possible to wrap it up with automatic deployment of the remainder of your forces.
The Yamauchi Uesugi, Kantō Kanrei title-holders, hold 6 castles in the Kantō region; their all-star subordinates include Nagano Narimasa, the brave Kamiizumi Nobutsuna, and the clever Sanada Yukitaka. Their downfall event does not fire if controlled by a player.
However, with the Hōjō, the Takeda to the west, and the Nagao to the north as powerful forces in the neighborhood, you'll have to act quickly lest things get difficult in the Battle of Kawagoe event.
(Example strategy: very hard difficulty, long lifespan, no new officers, no in-game edit)
Your first move should be to wage war against the Hōjō clan. If you're lucky, they should send out 12 units; consolidate your presence north of Edo Castle by way of Iwatsuki. A victory here should flip Edo, Kawagoe, Kozukue, and Tmanawa castles; this should thoroughly cripple the Hōjō. Also, as the Chiba, Oda, Utsunomiya, and Nasu clans should respond to vassal requests, it'd be good to vassalize them; as for the Mariya, since the Satomi will destroy them, vassalizing them is up to personal preference.
However, there's now a problem; while it's tempting to deal the finishing blow to the Hōjō, food supply has quickly become an issue. Being adjacent to the Takeda, reducing your military strength means increasing the risk of being attacked; it's difficult to avoid it via alliance, as they view you as enemies and already have 3 alliances.
From here, before doing battle with the Hōjō again, break your alliances with the Ogigayatsu Uesugi and Koga Ashikaga. Now that you have fewer alliance partners, begin goodwill diplomacy with the Imagawa so that you can form an alliance at the right time. (Again, the Takeda will be too difficult to ally, as they still view you as enemies.)