You can use the GIF (animated image) below or the written instructions for help on how to reach the area.
The rumble area is quite simple to get to. In the room/section of the area where there is a cluster of four plants, go upwards on the platforms until you see a Skull Switch above you. Run to the left, then jump up onto the platform there. Now leap as far right as possible, and if done correctly you should land on one of a series of steps/ledges. Proceed up them, grab the Barrel Rocket, and take off to the right, angling your flight path upwards. As soon as you see a ledge, jump off, as staying with the Rocket will almost always make you miss it.
Now, proceed up the flipping steps, and into the area above. Go to the left, and climb upwards onto the platforms to the upper left section of the area. Soon, you'll enter into another area, and the rumbles will begin. See the GIF (animated image) below to see how to get to the area if you are still having trouble.
Note that the last rumble consists of assorted Enemigos, not all the same type as the previous rumbles before it.
Counting all of the rumbles, there are: