Why not take your own Librarian Council, so you can Deny what they do?
Not a chance. A Librarian Conclave can *easily* push any given power through on 10 to 12 power dice if they really want, and chances are they're putting the four they want through on 3 or 4 dice each. 2+ to harness with a re-roll from Tigurius if you somehow fail it is bloody reliable, and the only powers they cast are Blessings so good luck Denying them.
I've been running this unit, and it's quite simple to deal with;
1. Ignore it. It's 24" range, 6" move per turn Slow and Purposeful. It costs 700 fucking points
to be any good. Often, it rolls up shit powers and can't actually function (if you don't get Invisibility/Endurance AND Perfect Timing, the build is a pile of shit and the unit just falls flat on its' face).
2. Get into combat with it. With, like, anything. It's terrible in melee for the cost. A horde of Fearless models is ideal.
3. Play the mission. They can't kill everything
, and chances are the rest of the Marine army is doing shit-all damage to you because Marines don't kill people to win games. Sit your butt on the objectives, in cover, and laugh as he kills some dudes while you blaze the rest of his fragile army off the board and win because he can't sit on the objectives as well as you can (Marines are usually king of this, but since he's got Centurions and a Conclave he obviously doesn't have the Battle Company built for OS).
4. Wait it out. At some point, this guy is going to roll badly on powers, and realise that this is honestly a shitty build that gambles hugely on a couple of dice rolls.
With the Centurion/Conclave star, I've beaten Khorne Daemonkin twice, lost to CSM badly once (they deployed out of range, shot the rest of my army to bits, and then charged the Centurionstar once it was the only thing left on the board - not hard when it costs a ridiculous number of points), twice to Necrons with CSM allies (shot up the rest of my army because Necrons are brilliant
at killing mech Marines, and then either ignored the Centurionstar because Necrons don't really care about 15 twin-linked Shred Ignores Cover Grav shots per turn or tied it up in combat with Orikan), and once to Eldar (lost some stuff to it, but then ignored it because I was way out of range of most of his army - on a 4x4 board at 2000pts no less - and given that Eldar have 12" movement on their slowest unit, he was easily able to simply avoid it for the rest of the game).