It took me a very, very long time to get this last achievement, even though I got all the others in a few playthroughs over a couple of weeks with no real grinding years ago.

I might consider writing some guides, since the existing ones are quite old and not as thorough as I might prefer. I never wanted to do this with 100 days still hanging over my head, but I finally cleared the game in 75 days. I've beaten the game many, many times with every difficulty level, choice of storylines, and fighting style, but the vast majority of my runs took around 110-115 days. Having finally cleared the game (and defeated the main character's brother!) in only 75 days, I now understand that the fame grind of the endgame was the part that was taking me way too long.

Here are some takeaways:
-100 days is plenty of time, but only if you are always winning fights. A stray loss is never a dealbreaker at any stage of the game, but it can't be allowed to derail your finances or your training. I got defeated once in the Amateur leagues, and honestly appreciated this hit to my ranking since it let me gain more skill points before facing Salvador Punchez. I got defeated once in the Ultimate Leagues, but I had plenty of money coming in and just treated the injury and resumed training like nothing happened. I got defeated once in the Dark Fist storyline, and just had some extra pizza in my diet and moved on like nothing happened. I got defeated once in my professional league fights for my pro card, the first time I made the finals. I took the L and just started over immediately, using the extra time to train. The guy who beat me (Nik Dartius) was my first opponent on the next go-round, and I beat him in one try.
-It is very important to be extremely powerful before you go to Russia. Going undefeated in Russia makes a big difference. If you lose a fight due to RNG, an extra few days won't kill you, but grinding skill points in Russia or Prison is not viable. you need a lot of skill points to get through Russia, so it's advisable to train with Silver constantly, and complete all available side quests before the midgame in Russia/Prison. I had Muscle Memory III and Tormentor Strikes/Exhauster before setting foot in Russia, and it made a world of difference to be able to focus on agility/stamina with 5 skill slots and no strength training.
-It's very important to be tightly choreographed on your first 5 days
-Skip Attack is overrated. I could not spare a skill slot or skill points for it. On a speed run, I recommend bypassing it. If you want to make the game easier regardless of speed, it's good for the early game. Regardless of play style, I always preferred the fastest horizontal track to lock my preferred primary stat to 8
-Bear is best for speedrunning. Turtle can become incredibly powerful, but as a defensive build it is very hard to deal meaningful damage without lots of time to train and gain skills. Tiger is also easy to beat the game with on any difficulty setting, but it's harder not to lose a lot of fights while you're building out a tiger. With very careful management of skills and stats, Bears can be extremely dangerous on the shortest timeline
-Health regeneration requires some attention on a speed run, so I recommend doing your Ultimate League fights one day AFTER your Amateur league fights. This gives you a day to sleep, eat pizza, and do just enough training to get your stats where they were the day before. You can always skip the Ultimate League if you're not ready, with only mostly a financial downside.
-Health permitting, I did my bar fights right after my amateur league fights, but didn't worry about skipping one if my health was badly damaged. Right after your Ultimate League fights is also OK, but only if your health is pretty close to full.
-When you finish the last bar fight, you're probably ready for the first Turtle, and should be a skill or two into your advanced style.
-It's probably best to face Bobo with a good 4 skill slot build, and a primary stat of at least 9, and whatever is considered a robust amount of stamina at your point in the game and with your build.
-The energy drinks and easy money from the trailer park/Bobo quests is essential for doing a ton of training in preparation for the pro leagues. Doing these as soon as you can be ready is a helpful strategy for getting ready for Mr Kekuse and Salvador Punchez

-I lost a stamina challenge due to some pretty bad RNG, and I lost a fight to Rio. On these two occasions, I verified that you can use Task Manager (I think it's called Activity Monitor for Macs?) to force quit the game from the results screen. This allowed me to reattempt the fights since the autosave hadn't saved my failure yet. I didn't spam this trick hard, but it did save my bacon when I got screwed during the game grind a couple of times. On a 100 day attempt, I would not be afraid to force quit and save scum in order to run back an extremely unlucky break. If I had abused this trick at all in the past, I probably would have made a <100 day run a long time ago. The RNG being what it is, it's easy to ruin an entire run because your character lost a fight by standing there doing nothing.
-The importance of getting 6 fight wins on the movies/gang wars is greatly exaggerated. I had a couple of 6 fight wins, but most of the time I had taken too much damage at 4 wins and just took a limited prize. I then got the remainder of my needed Fame from partying. Once you start beating world contenders, you get massive 4 figure payouts that let you afford to eat, train, and party.

I started the endgame at 13/10/10. I beat the game at 13/11/11. I would have loved another point of stamina, but the high agility was helpful in landing strikes on some of the late game speedsters.

My primary endgame build was:
-Knee Crush (same damage and cost as Boxing Punch; accuracy is marginally worse, but the debuffs to enemy AGI/STA are a nice hedge against various styles.

-Closeline (Never took this out of my lineup once it was learned; I didn't add a third skill slot until I had Boxing Punch AND Closeline, so I could quickly stop using expensive non-Bear attacks)

-Triple Punch (As soon as I learned Triple Punch, I used Closeline by itself and traded Boxing Punch for Triple Punch. This reduces my percent chance of spending my attack cycle doing nothing like an idiot, and reduced the cost of Closeline. I then added a 4th skill slot and reintegrated Boxing Punch. The first early game build I build as fast as possible is Block/Boxing Punch/Closeline/Triple Punch). Triple Punch is always great for its increase to your overall likelihood of attacking, and its cost savings help in early game. As the game progresses, its contribution to the likelihood of combos is essential

-Exhauster (This is one of the most diversely useful attack modifiers in the game for a high-strength build. Emptying your opponent's energy bar is the easiest way to stop them from doing something you don't like, and knocking them on their butt is one of the simplest ways to deal a ton of damage. If the opponent used dodges AND had a noteworthy speed advantage over me, even just 25%, then I often traded out Exhauster for Tormentor Strikes in hope of debuffing them so they'd be less difficult to hit. Switching my attack modifier is all the tweaking my build really required from one opponent to another.)

-Wreck 'Em (I do not recommend getting Ruthless Aggression until you have 10 stamina and Wreck'Em. Once I had both of these, I took the plunge and replaced Block with Wreck 'Em, which made it much easier for me to keep throwing bombs until some landed.)