It was a while but I remember. You use the flying koopa and Kooper (the shell toss dude) untill you can't reach the switch (or platform I forgot) then you use Lakiester (the hovering dude)to get through those obsticles then you go into the second hole and you fight a few koopas and a magi-koopa and hit the swithch that gets rid of all the lava and then get out of the hole and go into the other hole and get the key.
It's ah me Marioguy