Hey all,
 
I read about a few places that you can get a ton of credits in Rising mode, and figured you'd like to know one where you can get 200 million in one go (seriously).  Stage 5, there's an area with conveyor belts.  There's one section where one platform is going to the right, one's going to the left.  There are cranes and a place where robots drop (the one's that blow up and send pieces in the air.)

Hard to explain the location, so here's an amazing sketch: 
                                                                  Crane     Robot drop
 
 ______________>            <__ X_ shoot----> ____________________
 ^ platform moving right       ^ platform moving left

___________ platform with boxes_______________   pit     _____
 
 X is where I was at, and the edge of the screen was right behind me.  I shot to the right to kill robots, and up to kill cranes.  Got my multiplier up to 100x and was getting 300,000 points per kill.  Eventually they stop giving you points, but I had over 200 million and bought up most of the store.  It took like a half hour or so with an upgraded machine gun.
 
I beat the game legit and knew that there had to be an easier way to get the 57 million credit item (and that store achievement).  And above is my solution!

I think that shooting the boxes (behind you I think) is what keeps you from receiving the notice that stops earning credits.  I farmed some in Level 2 but could only get up to 40x points and probably got 15 mil each time.  The spot you found on Level 5 is way better and I saw on youtube that shooting the boxes behind you periodically will keep the credits rolling in.

I'd like to get credit but I saw this posted on Gamefaqs.  200 million seems to be a good number though, since I still had 25 million left over.

You can also go to the boss in the second part of the second level with the charge shot and continually shoot the alligators and bombs that drop from the helicopter.  This ranks up credits really quickly too.