The spell is supposed to cut attack in half; not completely nullify it. I used it to make the desert wildlands Boss (crusher of mountains...) more manageable. It appeared to work correctly against him, as he still had an attack strength around 40-50 post shrink spell. I had a big battle against the Trog faction later though, and it was way OP against the Juggernauts. Admittedly, I went ahead and abused it to turn what would have otherwise been a pretty rough fight into a cake walk (trogs are winning the game overall). Here is where I first noticed the issue: