After finishing the game and getting all other trophies I needed 8000 more orbs. Here is what I did.
Have all three of your character profiles on the highest difficulty each can do. In my case I had two insanity 1 and one insanity 3. Get each character right up to the point where you are about to turn in a mission and save. Between all three characters if you are on the same difficulty I was that is over 170 orbs.
At this point back up your 3 character saves to the cloud. Do not back up the profile save, just the character saves. Don't ever mess with the profile save, this is what counts the number of orbs you have earned.
After you have backed up the saves....quickly turn in all three characters missions to get your orbs. After the third character, quit to the main menu and then download and override your saves with the ones you have backed up.
Repeat 45 more times and enjoy your platinum.
On my ps4 pro with a solid state drive it took about 1.5 minutes per run(over 170 orbs each run). I did not think of using all three character slots until the very end(at first I only used one save which gave me 72 per run). This method should take most about 1.5-2 hours to work. Even faster if you are higher level and can set up even higher missions.
Hopes this helps some get this VERY slow trophy out of the way..