The mario bros stage with 50 coins is very easy. Go immediately to the left of the topmost ledge where coins will appear, run across, wait for fireball to come to you, jump over him, and then run under him quickly.
If you do it right, he heads right and circles back around to you, this is when you dash left, drop into the space and do a bomb, letting you jump the gap, collect those coins, run right, drop down to the ledge below, run left and drop to the bottom left most corner, wait for the fireball.
When it's close, jump up to the ledge, making sure he's below it, run, drop down, collect the coins in the middle on the very bottom, jump onto the last ledge, collect the last of the coins, drop into the pipe, and ta da! You got all the coins
Just takes some practice really, the fireball doesn't speed up too much, near the end it will look like he's about to get you, but as long as you keep moving, he won't.