Yakuza 0

Luckily, endless rout is a surprisingly easy battle mode, so this achievement isn't nearly as hard as it may otherwise appear. 98% of your time working towards this will be on grinding out coliseum wins and earnings in order to unlock the endless rout option. You need a "B" ranking in the coliseum, which is 200 mil in winnings, in order to move on to the endless rout, and while this may take some time, it's not nearly as bad as some of the other grindy achievements in this game.

The quickest way to get a B rank in the coliseum is to work your way up to the "Abyssal Romp" fight, which is the 8th fight and requires a D rank to unlock, and then use that fight as the grind. In this fight, the floor gives way in concentric circles and you just want to get your opponent to fall off as quickly as possible either by fighting/pushing them off, or simply avoiding them and hoping they fall off on their own which they often do with reckless abandon. Most of my rounds lasted around 10-30 seconds, so I managed to complete a full 3 round tournament in around 2 minutes with load screens. For the fight I used a nearly fully upgraded Kiryu and chose the bat option simply because I was comfortable with it. There's about 8 different enemies you can run into here, but the only 2 that really gave me any trouble were Invisible Hanzo and Vengeful Otake. Hanzo is annoying to fight with his array of attacks, but if you just keep your distance more often than not he'll just cartwheel to his death. I found aggressively attacking Otake immediately to be best, as his multiple gunshot attack will shred your health. Otherwise, I was making just over 7 mil a tournament and with needing 170 mil to go from D to B, you'll need about 25-30 attempts given a few losses sprinkled in. So some grind, but easily done in 1-2 hours depending on how much coliseum fighting you've done prior to going for this achievement.

Now, once you've gotten a B rank and unlocked endless rout you may be apprehensive about what comes next. I went in with no knowledge and expected something akin to climax battles, aka some garbage RNG and increasingly hard opponents. Luckily, from the time you unlock endless rout to the time you unlock this achievement should take all of 5-10 minutes. Upon starting it, you have a bunch of purchase options. Go with what you're comfortable with, but I found the basic hand to hand brawler for Kiryu to be best simply because it does a lot of damage and they're simple fights. Once you start, you'll face 5 random opponents in a row. These guys are not the same as the coliseum fighters and are laughably easier. I averaged 2-4 seconds per guy. Once you beat all 5 you'll be asked if you want to continue. Hit yes, you'll be given options for healing and then rinse and repeat 5 more times. Once you beat 30 guys, which is tracked on the endless rout purchase screen, your achievement should pop. I'm not sure how RNG dependent the enemy types are, but I had mostly the weakest olive green enemies with a few blue ones peppered in all the way up to 30. However, I kept going after 30 and immediately had yellow suited enemies who were beefier, but still manageable. If you get all olive green and blue enemies, you'll probably be done in a few minutes and confused as to why this achievement exists. Hope this helps!