Suppose the following. Boomerang is behind a fountain, which is behind a Heavy Object, which is also behind a Heavy Object, with a Firelord on a Heavy Object behind that. Here's a diagram:

xxxxxFFB
xxxxHFFx
LHxxxxxx

x=normal, empty spaces.
F=Fountain
B=Boomerang
H=Heavy Object
L=Firelord on a Heavy Object


Now, if I was to attack with Boomerang, does Firelord get +1 to defense only, or +1 for every piece of hindering terrain in the way?

It's only +1 for any amount of hindering terrain

Cool. Thanks! That helps me alot!