Krugman never "advocated" an alien invasion, he pointed out that when an economy is against the zero lower bound, government spending is not going to crowd out other money uses, and an alien invasion would prove it. Of course, so would infrastructure spending, which we can't seem to get.
Krugman has said a whole 'lot of things that were borderline naive at the very best and downright asinine at the worst. There are vast recaps of his stupidity, some on this very forum. Shall we go through all his comments for you to defend...er...I mean so you can "attacking specious arguments against them"? LOL!