The only way to completely avoid this problem would be to design a level so that there's nothing for the shell to bounce off of ever, except perhaps walls inside pits. That would skyrocket the amount of limitations placed on designers of any kind of level, and all for something easily preventable by the player.
It's common sense that if you send a shell speeding ahead of you, there's a strong likelihood that it's going to bounce back. While it can be difficult to dodge depending on slopes and whatnot, that's something you have to try and be ready for if you're going to take the risk. It's only one hit of damage in the end, and it happened because something went wrong that you as the player initiated yourself.
Cheez wrote:Yeah, it really depends on the layout of the level. Sure, you have to kick a shell to get it moving, but you also have to kick a shell to use it as a weapon. It's not really nice for a level to punish people who like to use shells as weapons.
The level isn't really punishing the player, it just happens to have Koopa shells, slopes, and walls at the same time. You don't have to kick it to use it as a weapon; you can avoid the risk entirely by running with a shell in your hand, then colliding with an enemy to cancel both it and the shell out. You can also hit a block on the ground without kicking it by holding the shell partway inside of it and letting go.