Well against what would most certainly be extremely advanced technology compared to ours just to allow them to come here in the first place, I'd imagine their craft to be superior enough that our weapons wouldn't stand a chance.
Making the only way either to get some pairs of boots ON one whilst its about whatever its about. Or use the creatures themselves against them. THEY wouldn't, I'd think be too difficult to squish individually, but do that and the opportunity is lost.
Just simple tactical logic. Why fight a nasty, drawn out campaign when you can use the people your fighting to do it for you, and wipe them out en masse with a plague. With man fighting man that is one thing, but in a global war for the survival of the human species itself there could never be room for doubt, mercy, ethics or moral limits, biological, chemical, tactical nuclear weapons, anything and everything we have as a race could, and if it would bring a tactical advantage or victory SHOULD be used, even if it were to mean kidnapping, torturing and/or killing their children.