The Kasrkin get a one in Malleus when some of them physically attack a daemonhost (read: daemon-possessed human capable of immolating them with, literally, a thought and able to shrug off laz-gun rounds). Granted, they caught it off-guard and said daemonhost was already seriously injured (missing its legs from the knees down, one whole arm gone and one working thumb on the other, and its neck snapped) but it still manages to kill a few of them. And they bring it down with knives and bayonets. Too bad daemons can Body Surf...
The Cadian Kasrkin in the Eisenhorn novel Malleus get one better when they unhesitatingly attack a daemonhost in hand-to-hand combat because the daemon is too close to a group of people they've gene-encoded their weapons to not fire at. Considering that daemonhosts are capable of obliterating entirearmiessinglehandedly, this is a sign of just how awesome the Kasrkin are.
Commodus Voke holding off Prophaniti. Just a Puny Earthling against a destroyer of armies, and although it cost him his life, it gave Eisenhorn the time he needed to destroy the daemonhost.
Uber Aemos summoning and later stopping Cherubael.
Eisenhorn's defeat of Prophaniti. Although he was using a weapon tailored to destroy daemons, this counts as a CMOA simply because Prophaniti was COMPLETELY destroyed, even back in the warp. It should be noted that this is an insanely rare occurrence.
In Hereticus, Eisenhorn is called by a close ally to a meeting. However, it's a trap and Eisenhorn is surrounded by a minor villainess and a gang of well armed mooks. Even he can't worm out of this one...except he's over 3 miles away, mentally controlling a psychic puppet bomb. After getting all the information he needs, he then wipes out the whole party.
End of chapter 1 of Xenos:
Come and let me show you how I killed Eyclone.
This is before he shoves the pistol (customized by Vibben, who just died) into Eyclone's mouth, shattering his teeth ,and then PUNCHING it into his skull, driving the finger guard into his mouth before blowing the back of his skull out.
Cherubael frying a Chaos Titan, doubles as CMOF as a mad priest with an imperial eagle plaque then makes him scream like a girl.
Cherubael's curbstomp of a fight against another Daemonhost at the end of Hereticus after being sorely restricted in power by Eisenhorn while his opponent is barely bound to his host and therefore able to throw everything at him, while Cherubael isn't even allowed to laugh.
Cherubael:"You have no idea how strong I truly am, Eisenhorn."