Played against a Keeper of Secrets once.
GM had sent us into this network of underground caverns. A band of Emperor's Children had set up some sort of daemonic shrine/structure, and we had been given a set of strange devices which we didn't know how they worked, only that we needed to put one at each corner of the structure. We fought our way down there, past loads of Noise Marines, and eventually found the structure. We got the devices in place, and that set up a warp-field around the building, but not before the Emperor's Children managed to bring in some Daemonettes and a Keeper of Secrets. The devices also acted as beacons, and a small squad of Grey Knights suddenly appeared to help us out.
Well… that turned into a cavalcade of fail, as the Grey Knights wiffed their attacks almost every turn, and did nothing. Eventually it came down to my Sword Brother and all his anti-Daemonic abilities to bring down the KoS, whilst the rest of the Kill Team dealt with the Daemonettes and remaining Noise Marines. I had about 3 GK's "helping" me in that combat, but most of them just acted as extra wounds their rolls were so bad.
Ha! Nothing beats the will of the dice, even with the best intentions. You say you killed lots of Noise Marines - you must have been fairly high Rank, then?
I had my Kill-Team encounter a Keeper of Secrets in the bowels of a Space Hulk. Between the KoS's ungodly dodge save and my Kill-Team's collection of field saves, dodges, and parries, along with a decent dusting of whifftastic rolls, I think it took three rounds to even lay a finger on the thing. Then the Devastator landed a clean shot with his assault cannon, the Librarian channeled Smite, and everything lined up. In short, a Greater Daemon fight really depends on how you set it up. I guess the first question would be, what rank is your Kill-Team?
Hmm, another Keeper of Secrets huh. As I recall from Second Edition, they were kind of the "weakest" Greater Daemon - is this also true, or is this just a holdover perception from the wargame? What Rank were your Kill-Team?
As I allude in my original post, I won't put this into the game at a Rank that I don't think it'll make a good challenge for the Marines, so it can wait until then.