Mario DS: Help thread

[GameMaster]

I have 49 stars and the only characters I have are Mario and Yoshi. Should I have already got Luigi and Wario at this point? If so, how do I go about it?

My other question involves the level with those stone slab guys. The last mission is titled something like 'Shoot the Wall Down' and I have no idea what to do for that. I'm thinking maybe I don't have the right character yet.

And last for now, any advice for the that water base level with the mission titled something like 'Elevator Express'. It's where you have to step on the elevator thing and then it lowers down and then you quickly have to run down under it into the caged area and get on the inner part of it to get back up to the star. It's so hard to do because of the camera angle.