After defeating all eight Mavericks, go to a stage that you can get to the teleporter fairly quickly and easily (such as Infinity Mijinion). Get to the Boss (not the main one, the one where you fight Zero or High Max). You should find Dynamo there. Get next to him and use the Meteor Rain attack (Ensuizan for Zero) twice. Dynamo should drop a Nightmare Orb worth 200 Souls. You can make him do this three times before he goes away for a total of 600 Souls. You can do this as many times as you needed.