When you first visit the advanced martial arts dungeon, you're probably thinking that the shop keeper and guards are way too strong to handle. When you get DEF to being at least 3 million (3.5 million preferred, 4 million for best results), shop guards are easily handled. This means, with enough ATK to do damage (1.5 million minimum), you can defeat them. With all 6 sources directed to demon soul, you can get 200K GP each guard. In this case, I only have 2 sources directed to it so I'm only getting around 106K GP per guard. Either way, I gained a ton of GP doing this, and a ton of level ups. I throw in a 5x time lapse so you can watch the level and GP climb at a fast rate. For best results, use a skill that uses little SP so you can keep at it more and blast more guards for more GP and level ups.