...snip science ...
Yep. But we thought they were impossible on biological grounds, not on physical ones.
Could we back the truck up and confirm that everybody's talking about the same scale of monsters?
I haven't seen Pacific Rim. I assume the traditional Tokyo destroying monster is about the size of a Dinosaur (t-rex or allosaurus or whatever the big scary predator is called this week).
Clearly, we can get Dinosaur sized monsters that biology and physics can agree could exist (because they did).
So are Kaiju assumed to be even larger than a really big dinosaur? Or should we assume when somebody says Kaiju, they mean something big like a dinosaur that is totally possible?
If the former, then of course we enter the mystery physics land of "that's not possible to exist, so if it does, it is extra harder to kill because there's more complex materials at play.
If the latter, then as we learned in Marvel Ultimate Alliance, nothing is immune to bullets. A modern military most likely has the ordnance to take out a dinosaur-like monster be it 50cal rounds, missiles from jet airplanes or bombs from bombers. Given that a man can throw a harpoon and penetrate a whale's thick skin, the upper end of our weaponry can pierce and kill darn near any biological entity.