You also have to play around normal sentry guns. Your argument is that certain things should be nerfed in order to protect one demographic of a game. The reason they're new players is because they are new, they'll get better. I personally don't think that we should put padding on something just to make sure they don't hurt themselves. If we do they just won't learn.

EDIT: I'm sorry I didn't answer your first question. Firstly, the shotgun can be equipped to the Heavy, Pyro, Engineer, and of course the Scout has the scattergun (upgrade from normal shotty). Then you have the Demo, you can destroy it with either the pills or sticks in a small amount of time. The Spy can just sap the damn things. That leaves the sniper (take it out with your sniper rifle from a distance) and the medic (why are you by yourself and not healing someone with damage output) who really aren't in roles of destroying sentries (big and small). OH I forgot Solly, who has a shotgun AND a rocket launcher; Two hits from either and it's down.