https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2297620602

This trick is detailed elsewhere multiple times, but I've added a little twitst.

If you go the homeless camp, there's a storage box. Right around the corner there's a couple of red vending machines that sell Kiwami drinks - They will boost XP gained by 0.2.
Get a couple of Yakuza Training Gear and Tranquil Tenugui (You can get one from the Vocational School, and I found my second one on top of the Mahjong parlor in Sotenbori - Along with an Job XP boost for Kasuga) - For extra Training gear you'll have to use the batting cage >.>

After loading up on Kiwami Drinks and gear, go to the Kamurocho Underground and reach level 2.
Do so by skipping every fight to save time - The battles aren't worth the time investment.
When you're at level 2, the 3rd encounter has an Invested Vagabond.

Personally I have the Job DLC which gives the girls the Matriarch job, where one of the skills is Bullet of Honor.
Sometimes it'll kill the invested vagabond "instantly" but when that fails I use Joon-gi Han's Rapid Shot.
Dragon Decimator with Zhao is another skill that does a lot of damage to a single target, so if your setup/party uses him, try hitting him with that (Credits goes to AnotherZero for the tip).
In a pinch Twirling Beat is another skill that does multiple hits to a single target.
After killing the group, head back to the top (where the save point, and storage box is) and grab more Kiwami Drinks then rinse and repeat :)

Here's a short video from Weaven showing the process, and how it can be done (Most important to note is the route).
Again: If you don't have the Job DLC, use Joon-Gi Han's Rapid Shot, or some of the other suggestions in the comments!
https://youtu.be/s2bZOlPP39I