Well, got my five shiny stars in NSMB2 today. ...with just under 100,000 coins.
And Nintendo wants me to do that nine more times? Eh, no thanks. Even at 7k a pop in World 2-4, that's still pretty darn insane. If they wanted to bank on people playing the game's multiplayer forever, they should have made it online. At least with a console game you only need one system for multiplayer.
Guess for handhelds I'll be going back to Mario Kart 7. I still need to try and do the whole Kid Icarus on Intensity 9 thing, too.
Coin Rush.
Mushroom Pack, making sure Mushroom-1 is the first stage.
Gold Flower.
Run and hit the checkpoint, use the earlier enemies to get around 500 coins.
Go into the pipe where the first Star Coin is, Run and jump onto the koopa twice.
Break the bricks to form a small set of stairs.
Infinite 1-UP trick, making sure to stop at about 900 coins (The Koopa only has 1,000 in him, and if you take them all then he dies and doesn't respawn.)
Do that twice more, third time feel free to kill it.
Run to the end.
Top of the Flagpole.
Next two stages don't matter so long as you get the top of the flagpole in the second stage (The third is always a fortress IIRC) to double your score, just killing the boss in a Fortress produces the same results.
You'll get the maximum 30,000 for Coin Rush, save that as your record so you can always Challenge it to get more.
I got to my 1,000,000 within a week thanks to that. I'm not going to bother with 9,999,999 though, not worth it.