In response to Mastamindkar-
IT'S A SPOILER!AHH!AHH!
Those knights weren';t armed with swords. They had blast lances. IE- Lance Tip hits target, Lance go BOOM. It's a focused boom too. And only one Grey Knight dies.
The Grey Knight SQUAD- ONE SQUAD- proceeds to sack an inteir city, defeating it's inteir resident army and carving their way through, defeating the gigantic mutated king, and then they killed a Chaos Sorcerer of tremendous power.
And the scene at the end of the book, most of the Sisters of Battle died outright, out of a small army. Only a few made it to the actual demon. And this was by the point the Grey Knights had cleared the road and were "distracting" the demon.
And then the Justicar OUTTHINKS a daemon of Tzeenitch.
Plus your forgetting the beginning of the book, where 3 or 4 Squads of Grey Knights take on an inteir Daemon world and win. Of course, they do this by killing their leader, but still...