There's various methods of grinding souls. The easiest would be taking +5 storm ruler at the 0 sen enemies in crash chamber. Crank the difficulty as high as you can as long as you are one shotting them. If you have rabbit keys, the rabbit area is good too. After those would be fighting Miranda. At as high a difficulty as you can win by mashing z. Then there's the chaos dungeon. The main dungeon is alright but the real stat grind is the boss rush. Doing the full thing gets you like 18 million souls (with double drop from prickett or silver ring) + you get like 30 outer one souls as an additional reward at the end. You end up getting around 1500 stats per run after converting the souls to black souls.How are you guys boosting your stats so high? Are you just collecting obscene amounts of souls and buying the colored ones from Node?
You don't want to buy colored souls. Buy black souls from either Hein or Mabel. All types of black souls have the same stat per soul spent so just buy whichever.