How do you blast down the wall on the island in the sky level?

Well, if your talking about whomps fortress.
Climb up the temple, past the whomps, past the sleeping petty piranah, walk across the falling bridge, instead of going on the moving bridge, jump down the slope, talk to the bob-omb buddie and he'll prepare the cannons, go in the cannon, then fire into the corner wall, the one with the bridge in between it, hit the one on the right.

Hope I helped.

OK this is what you have to do:

Near the wall there is a cannon (if you havent open it, open it)

Jump into the cannon and shoot your character against the wall

If you hit the wall at the right spot, you will break it

If you mean the second level with the chomps, heres how: get in the cannon with anyone, and turn till you see the wall by the chompes and stuff, its a skinny wall(the one close to the mario hat if your yoshi), and blast your self at the corner, once the wall is broken the star is right there.