Like a Dragon: Infinite Wealth

Durign the story, you'll catch up with Professor Morikasa and begin the Sujimon substory. Completing all Sujimon substories means taking on all of the so-called 'Discreet Four', starting with Jack and his underlings at the Regal Jack Resort. You'll then have to take on Queen, Joker, and Ace. To take on Jack, you need only level up your Sujimon to a maximum of Level 10. Taking on Queen requires your Sujimon party reach Level 20, but only by defeating Jack can you attain that level cap. Once you've polished off the Discreet Four, you'll then have to fight their Champion, King. Once you've vanquished him, you'll wrap up the series of Sujimon substories. Nice work!