Idea generated in the discussion here as a way to make life more interesting for magic using characters. Essentially, I think it would be very cool and thematically appropriate if living walls and wall beasts (and maybe one or two other rock-type monsters like Ancient Stone Beast) should behave as walls as far as bolts are concerned. So if you attack a living wall with lightning bolt, it bounces back; if you attack with fire bolt, it fizzles out. I think it would be fair enough for this still to do damage to them, though this could go the other way if you wanted to really up the danger.

While these monsters are already pretty powerful, I don't think it's a huge problem balance-wise considering that these monsters are extremely rare anyway and the effect (especially if it still damages them) doesn't necessarily make them more dangerous (except in groups), just more unique.