In The Black Throne, on the way to the Third Guardian, you will approach a bridge with spinning portals (The Black Throne, The Third Guardian III). When you get across, you will first be ambushed by a Rot Mauler and a Shield Lord. Defeat them and your next opponents will be one Shield Lord and four low-level soldiers. Do NOT kill the Shield Lord. Instead, keep killing off the four soldiers and they will keep coming back, giving you as many souls and as much experience to max out your weapons as you want! To make things easier, run away from the Shield Lord whenever he gets close, and the four soldiers will follow (they are faster than the Shield Lord so you should be able to get some hits in, run away when the Shield Lord gets close, hit the soldiers some more).

During the Broodmother Boss fight you might notice you get souls for destroying the egg sacs by throwing them at the ceiling. Simply defeat the Broodmother, DO NOT LEAVE, and keep grabbing egg sacs and throwing them at the ceiling (They keep respawning) for unlimited Souls.