Up to 550K in about 15 minutes.
I am adding this guide because I know a majority of the world hates this game (Trust me I do to, but I still think it's playable).
On mission 9, with Dante, run to the room with the lava pit. Jump on the ledge above to spawn all the bugs. Make sure you have the SMGs equipped and jump in the lava. Just roll back in forth until your stylish rank finally reaches S (Showtime). Then just hold down the trigger on the SMGs and they'll all drop like flies. With an S, each bug will drop a green orb and a white orb. Each white orb will be worth 500 red orbs with a full DT gauge. All in a span of 15 minutes you'll have up to 550K red orbs. Just be sure to save before the timer runs out and restart the mission.
This should help you blaze through the achievements so you can get onto much greater things, like DMC3