ok so I can't find the answer to this one anywhere under Daemonic, From Beyond, Strange Physiology or what have you ... so I am wondering: can Daemons be stunned?

And are these rules the same in DH1 and DH2?

I had a DH2 Priest go mental on a Flamer of Tzeentch with a mundane hammer, stunning it and taking it out way before any dramatic combat I had in mind could unfold

I believe so. Immune to fatigue but can still be stunned.

Makes sense imho. Bash a daemon in the face with a suitably powerful weapon and even something like that might be dazed for a moment or two.

Of course actually stunning it would require a great deal of luck, as besides the takedown talent, the only reliable way to stun a daemon would be to get it into crit. anyways. The scary thing about daemons isn't how resilient they are, or how powerful they are, or anything else imho besides the fact that you go mad and roll on the shock table just for seeing one if you can't pass the -20 Willpower test.

The trait "The Stuff of Nightmares" grants them immunity to Stunning, among other things.

But the only warp entity with Stuff of Nightmares is Adult Psychneuein, in Corebook at least.

A mistake, all daemons in later editions have the trait.

Deamon Prince in DW, Veiled Deceit in RT, Chaos Spawn & Daemon Prince in OW - all others don't have SoN. I don't own DH2 and my BC materials are far away so can't check them.

And no Chaos Space Marine has Astartes Implants and Grey Knights have no rules for repelling Daemons or even Psy Rating.

Don't take the FFG NPC profiles at face value.

Well, I personally don't make daemons vulnerable to stunning, but we are in "Rules question" section, not "House rules" one, so I stick to RAW.