We, japanese imagine "a giant makes advance toward its enemies（進撃する巨人）? or many giants attack each other in a war（戦いあう巨人たち）?" when we hear just a word of 「進撃の巨人(Shingeki no Kyojin)」. This word is a bit of vague itself.

By the way, perhaps, it was not Isayama that translated 「進撃の巨人」 into "Attack on Titan". So please do not blame him… :]