This one will probably be a bit of a stretcher, and doesn't hit the rarified lows of the Spengo, Psychlosm or most YA novels.
However as runner-up, I'd nominate humanity in the John Ringo Legacy of the Aldenata series.
The Posleen are immune to chemical and biological weapons. Further, they are mostly sub-sentient morons (A few are "God-Kings" who have human intelligence and lead) barely capable of operating a rifle and many of their engagements wind up hand-to-hand using monomolecular swords that can cut through any armor humans possess. They eat humans and multiply explosively, food is the biggest limiter so getting at a city full of civilians almost immediately gives them a huge population surge of new soldiers.
The big thing is the Tenar, basically tiny flying saucers with near-perfect automatic point defense the God-Kings ride around in. A Tenar's point-defense can be saturated with enough fire and further, a Tenar will not fire on anything that's on a ballistic course, it only shoots things that are guided or powered in some way.
Looking at this setup, mankind decided to go with highly expensive power-armor and try to fight the Posleen hand-to-hand (And, I guess, just hoped the Tenar would go away or something since the suits had no way to deal with them) instead of, y'know, building so many cheap tube artillery and easily portable mortar platforms no Posleen ever finds out what color the sky is. Further, they decided the best option to engage the Posleen right before the hand-to-hand stage was ridiculously high-velocity bullets instead of, say, flamethrowers.
It gets worse. The Posleen don't understand water, have no (water) ships or submarines, and even rivers are serious barriers for them. So when deciding what to do with all the evacuated civilian populations, naturally the humans built massive floating/underwater cities that... oh, wait, they instead built ridiculously expensive underground bunkers and disarmed all the people inside so that the second a Posleen found the place it became an all-you-can-eat buffet that would immediately generate a Posleen population explosion. Humans also scrap all their ships and such because obviously having offshore artillery platforms the Posleen can't touch aren't going to be any use, right?
