When you run up the planks of wood to get onto the roof, you will fight three 
enemies. Defeat the enemies, but do not fall down to the room below. Simply kill 
the enemies, jump back to the planks, then jump back to the roof. As you land, 
immediately double charge your weapon of choice, and kill the enemies again. 
They will continue to respawn as long as you do not jump down into the room 
below. It is possible to get 120,000 Yellow Orbs in about two hours.