OK, I finished my first 3KTW campaign!!!

Zheng Jiang has a great "+50% income from tributaries". So I was making vassals left and right. It was a bit of an issue to keep them not-unhappy some times but it was doable.
In short, after some time that I had him practically boxed in, Lu bu (he had taken over Dong Zhuo's faction after the death of Dong Min) Declared War on me while I was cleaning Yellow Turbans from the Northwest. That was not exactly a surprise so I had the armies and cash at hand to deal with him without issues. Honestly, the Yellow Turbans were still a greater challenge than Lu Bu. The YT had crap army at the time, but endless hordes.
Anyway, while dealing with those, I sneaked an army + 3 heroes aaaaaaaaaall the way to the southern point of the map. Why? There were some looters there. And their provinces ended RIGHT NEXT to Cao Cao's Seat (he is an emperor). Cao Cao was consolidating his empire to... an Empire, had the Han Empire as vassals and a few others and some Imperial subjects that were next to my borders. It would be a looong war to go down there the honest and direct way.

By taking the provinces not-exactly next to his, I could bring 7 armies, Seven FULL armies. Eventually, I took the province next to his capital and then ... the "issues" started. DOWs from a few friends of Cao Cao started showing up. Deals falling through. Etc. I don't know if it was Cao Cao or his friends were really pissed (since I was a huge, aggressive faction competing with their Emperor for who is the real Emperor), but diplomatically, it was heading towards a "Realm Divide" situation, sooner than later.
But...
when I made my move to take the province next to his I had the armies there already. Thus, barely 3 turns after I had a border with him, 3 armies crossed the river, 2 went up from the sea. Zheng Jian (now level 5 and with everything gold) and a bunch of 7 level heroes with a lot of silver and gold stuff, led those armies to break a bunch of agreements and declare war.
I got his capital in that swift, treacherous strike and because of the 3KTW gameplay, if an Emperor loses his seat, his Empire dissolves and he is downgraded to king. Thus, with that one sudden strike, Cao Cao lost his Empire BEFORE he could act and call his allies to war.

Not that it would matter anyway...
When Cao Cao's capital fell to Zheng Jiang I had all three seats. Through vassals and my own empire, I owed 106 settlements. Well, the "Unification of China" is at 95 settlements. Which means... I won the campaign the moment the capital fell.
I may continue Post-victory, but really, I have 7 armies in his undefended heartland, loyal vassals to block his armies from reaching me. And with a little money, the "Han Empire Holdings" readily agreed to become my vassals. Following that win, his former Imperial subjects don't seem to have ill will towards me. I am the legitimate Emperor after all. Thus, it is Cao Cao and his remaining vassals. Unless I buy off those vassals. Thus, I can very easily crush Cao Cao.