If you took the secret exit in the 7-Tower, you unlock 7-6. The main enemies are Para-Beetles and Heavy Para-Beetles. You need to ride on these beetles to cross the level. A Yoshi is in a brick to help you move to a higher parabeetle. Or if you are Propeller Mario. Ignore Yoshi and you can just fly from one parabeetle to the next. Standing on Para-Beetles rise you upward while the heavier ones just descend. It starts on land but then gives way to a very wide gap.
Hint:Standing on 8 parabeetles or mixed with heavy parabeetles will reward you a 1-Up Mushroom.
Tip:Beating this level grants access to a Gold Mushroom House and a secret entrance to 7-Castle where you will start in front of the Sledge Bro. and the double doors leading to Ludwig Von Koopa.