How do I get an excellent attack with a shell?
Solution 1:
In order to get an excellent rating with a shell you need to press A both times Mario jumps on the shell. The first time being when the koopa is walking into the battle, and the second jump being the one which launches the shell. If you only perform one of these jumps you'll get a nice rating.
Solution 2:
With a shell, it starts to fold - this is where you get the Nice rating. When it is till folded, right before it launches, hit A again, and you should get an Excellent rating.