As seen in Hudson222's any%/manga% runs and each of my own runs, there's a way to exploit the enemy AI to force them to not get up after the first down. Each punch in the game deals a combination of red damage and yellow damage (with the exception of jabs which only give yellow damage, and body blows which only give red damage). If the enemy takes less than 140 red damage, they'll get up on their own, but if they take more than 141 red damage, they'll use their spirit to get up. But certain punches do fractional damage, meaning if we can do between 140 and 141 red damage, they don't get up at all. With a lot of help from Hudson222's spreadsheet, I've found consistent, easy 1-down KOs for everyone in story mode except for Miyata, Hayami, Date, and Sendo 2. Shown below were my ideal combos for everyone in Manga%.
Miyata: Dodge punches whenever possible, and land 5 hooks followed by a Strong Upper (Up+R+B). Two of those will give you your first down. For the second down, if you get it in early enough, you can down him with just 5 hooks. Avoid ending all 3 combos with a Strong Upper, since it will deal enough damage to make Miyata use his down-avoid upon getting up. For the third down, just land 5 hooks and a Strong Upper to take the win.
Hayami: Similar to Miyata, land 5 hooks and a Strong Upper. Try to get the next combo of 5 hooks in asap, since if you wait too long you'll have to end it with a second Strong Upper, which would put in him down-avoid range. Once he gets up, you can finish him in a single 5 hook + Strong Upper combo.
Mashiba: Now we have the best move in the game: the Liver Blow. It deals a shitton of red damage, which is perfect for us. Land two body blow + liver blow combos, and follow it up with an uppercut + Liver Blow combo. Open all non-hook combos from here on out with jabs, since they deal 0 red damage and are an easy way to gauge if they have their guard up or not. It also prevents your punches from gaining a 1.5x multiplier from being a counter. Long story short, if you dodge a punch or your enemy fails a dodge, the first punch of a combo gains a 1.5x red damage multiplier. We don't want that.
Sendo: Same as Mashiba. Exact same combos.
Okita: Much like Mashiba and Sendo, lead with 2 sets of body blow + Liver Blow combos. Follow this up with a set of 5 hooks + Liver Blow, and he'll stay down for the count.
Saeki: Same as Okita. Do the same thing.
Volg: Lead with 5 hooks, and follow it up with 2 sets of body blow + Liver Blow combos. End the fight with a combo of 5 hooks + a Liver Blow and he'll be doneskies.
Date: If you're doing an Any% run, just throw out a lot of hooks + liver blows until he goes down 3 times. His damage multiplier means there are almost no punches that do fractional damage, so it's probably quicker to just fight normally. If you're doing a Manga% run, let him give you hardcore brain damage and just be a punching bag until you go down 3 times. Get up asap to limit the amount of time you stay down, use instant get-ups if you have to. (Up+R+A)
Shigeta: Same combos as Volg.
Sendo 2: He has the same damage multiplier as Date, so you're going to have a really hard time landing a 1-down KO. My best plan was to throw hooks + Liver Blows as often as possible and just get the standard 3 downs.
