I once used a sewer full of Myconids (replete along with a few pipes that the PCs could climb) that were indestructable unless you bounced on their heads - twice! -the first time you bounced on their heads they would shrink down to a foot high, the second time they went squish. By defeating them the PCs got a pair of magic hammers.
(other nasties included turtlemen, crocs, ba-bombs etc)
The BBEG was a giant Turtle-man (I forget the race name) who had kidnapped the princess. His shell was impervious to all damage except for that caused by the magic hammers (which also cast 'earthquake' with slammed against the ground)