Easy way to farm red orbs. Play secret mission "taste of heaven" one hit one kill, hold RT + press B ,dont kill flying enemies, about 200-300 red orbs in 10 sec
To unlock Taste of heaven: (info from http://www.gamefront.com
"On any difficulty, go to Mission: 8 (Eyeless) and get a bronze key so you can enter the Secret Mission: Taste of Heaven.
The secret mission door is located on the right side of the long hall that generates after using Demon Pull on a wall.
Look on the righthand wall of the new path for blue vines blocking an entrance. Use the Aquila or Osiris with the Double Up ability unlocked to get through the vines."