Thanks to Baja646 we know that enemy weapons can be manipulated by limiting what you get in Weapon Master. With this in mind to get better RNG it is advised to have a file set up for it. Real useful for good runs with Maxi Extra Arcade or Charade runs so you get good weapons for him.

Ideally for Xbox One create a profile dedicated to this with the save file using this. On your main profile you can 100% it if you want to. And another profile to use that will not save any data by using the don't save option at the start. Not sure about PS3 or PS4, but I would imagine the same create two other profiles along with your main one.

Extra Arcade Spawn Route 1: Complete Weapon Master as fast as possible only the missions needed to progress nothing else. 2: Buy only these weapons. Buy Cat Claws in Chapter 1 Buy Morphos in Chapter 2 Buy Double Crescent Blade in Chapter 3 Buy Spine Blade in Chapter 3 Buy Tenkamusou in Chapter 3 Buy Zantetsuken in Chapter 3 Buy Giant Butcher Knife in Chapter 4 Buy Wo Dao in Chapter 4 Buy Viper Edge in Chapter 5 Buy Fire Blade in Chapter 6 Buy Nanbanfu in Chapter 6 Buy Bardiche in Chapter 7 Buy Amud in Chapter 8 Buy Masamune in Chapter 8 3: Complete Sub Chapter 3 Pollux (Don't have to beat the dungeon twice only once) 4: Go to Subchapter 4 Procyon and complete Stage 1 (Mulphein's Crypt), Stage 3 (Mount Procyon), and Stage 4 (Lynx Arbor). 5: Go to Cepheus Shop and buy Abomination, Vajra, and Lethe. 6: Complete Arcade with any character to unlock Extra Arcade 7: You are finished and can do Extra Arcade runs.