If you don't get this through normal play you can farm red orbs in mission 13 with dante super easy. Just make it to the divinity statue and equip Faust, preferably upgraded to use and make more red orbs so this goes faster. Jump down to the next floor and use the normal gunslinger attack with Faust along with the follow up detonation attack. This will rack up red orbs pretty quick and you can restart the checkpoint once you kill everything to do it all over again.
Heres a video detailing the farming method.