Well even if you don´t know their weakness they are what....as big as a 2-3 story building? they appear to be that big on the picture. Most missiles would blow them totally to bits even without knowledge of the weakness. If they appear randomly it is quite impossible to debate the first few days but i think most militaries would organize quite fast and then bomb them to oblivion.
This is the size range for the Titans.
Really, the only way Titans would be anyway of a threat is if they're all humans who retain their knowledge in titan form.