A Battle Group idea that I have been batting around is as follows:
-6 man UM Assault Centurion squad with Sgt Upgrade, TLed meltaguns, and Hurricane Bolters
-3 man UM Libarious Conclave with Tigurius and 2 x TDA/SS/Force Axe Librarians with PML(2)
Assault Centurions provide durability (12 x T5 wounds with 2+ armor), anti-tank/anti-MC (meltaguns shooting and Siege Drills in assault), and anti-horde (36 TLed Bolter shots at 12″). Tigurius provides great Psychic support (more on that later), the Storm of Fire WL Trait (making all those Hurricane Bolter shots Rending!), plus the ability to force multiply the rest of your army with his Reserve re-rolls. Finally, the 4 TDA/SS/Force Axe Librarians provide Warp Charges, NUMEROUS chances to roll on the desired psychic disciplines, and the ability to tank for the Centurions against AP2/1 attacks with their Storm Shields.
So, how does all of this work? Well, the key to this “Assault Centurion Deathstar” is of course Psychic powers. LOTs of Psychic Powers! Between Tigurius and the Librarians, this battle group is generating 7 powers (not counting Primaris power from Psychic Focus) each game, with Tigurius able to re-roll to have a better chance at getting his preferred 3 powers. Additionally, based on the rules of the Libarious Conclave, Tigurius can use ALL of the powers from the entire formation and harness WCs on a 2+ (re-rollable thanks to his Hood of Hellfire) as long as he is within 12″ of the other two Librarians and they are not used to harness WCs.
Summarizing, this means that Tigurius and company have a very strong chance of getting at least some of the powers they are aiming at, plus an almost guaranteed chance of successfully casting those powers (he has a 97% chance of successfully harnessing each individual Warp Charge). Now, how do all these powers support the Assault Centurions? Let’s take a look by each available psychic discipline:
1 – Divination – Prescience for re-rolls to Hit in close combat, Forewarning to give the Assault Centurions and Tigurius a 4++, Perfect Timing to allow weapons shooting to Ignore Cover, and Precognition to turn Tigurius into a close combat champ are all great options from this discipline for the Centurion/Conclave Battle Group
2 – Pyromancy — Fire Shield gives everyone a 4+ Cover save in the open, Fiery Form gives Tigurius a 4++ and basically Shred for all the Pyromancy Witchfires, plus a whole bunch of “crowd control” Witchfires to help the battle group deal with hordes makes this a decently useful discipline to roll on.
3 – Telekinesis — Not the greatest options for maledictions and witchfires, but Levitation addresses one of the key weaknesses of this battle group (i.e. lack of mobility) with a free 12″ additional move in the Psychic Phase, Telekine Dome gives the Centurions and nearby allies a 5++ against shooting, and Psychic Maelstrom provides an “uber psychic demo charge” to Tigurius every turn, making him a real threat to anything T5 and below with its S10 AP1 Large Blast.
4 – Telepathy — Invisibility of course is a winner her, but The Shrouding is solid as well for giving the unit a 5+ cover save in the open or a 3+ cover save in basic terrain. Additionally, Psychic Shriek is a great Primaris witchfire against tough opponents and the other Blessings/Maledictions can all support your army effectively.
5 – Biomancy — Some useful Witchfires in this one, plus the ability to make Tigurius into a monster stomper with Iron Arm and/or Warp Speed, but what really shines here is Endurance (for EW and FNP(4+) on the entire unit) and Enfeeble (making enemies -1T/S, which amps up the offensive and defensive capacities of the Assault Centurions multiple levels.
6 – Sanctic Daemonology — While the dangers of perils increases here, Tigurius’ ability to harness WCs on a re-rollable 2+ means you can reliable cast almost any power with minimum dice, in turn minimizing the chance of a Perils. Banishment is only good again Daemons and Hammerhand doesn’t really help the Centurions (they are already S10 AP2 and striking at I4), but Gates of Infinity for mobility, Sanctuary to give the SS Librarians a 2++ for tanking purposes, and the three witchfires are all quite solid.
7 – Librarius — One of the best options, IMHO. Emperor’s Wrath and Fury of the Ancients are decent witchfires and Psychic Fortress/Psychic Scourge provide solid buffs to your ability to beat enemy Psykers, but the real winners are Might of Heroes (making Tigurius into basically a Lord of War), Veil of Time (Centurions can re-roll their 2+ saves and Librarians can re-roll their 2+/3++ armor and Invul saves), and Null Zone (reducing enemy Invuls by -2 makes Assault Centurions even more potent in combat, especially against enemy close combat characters like Lysander, Calgar, Abbadon, Greater Daemons, Dreadknights, Wraithknights, etc.).
8 – Fulmination — Again, some decent witchfires for crowd control and Electroshield providing a 3++/Fists of Lightning providing more close combat power to Tigurius, but where this shines is Magnetokinesis and Electrodisplacement powers, which allow for almost OP levels of mobility, especially with the ability to move the unit up to 18″ for “free” or switch its position (and charge afterwards) with another friendly unit.
9 – Technomancy — Most of these don’t apply to this Battle Group doesn’t have any vehicles, but Warpmetal Armor can make the unit majority T6, which basically turns the Centurions into mini-MCs and VERY hard to take down with anything but heavy anti-tank firepower or Grav Weapons.
10 – Geokinesis — One of the less useful disciplines for the Centurion/Conclave battle group, but Shifting Worldscape allows the unit to “ride” a piece of levitated terrain up to 24″ around the board, making for basically an improvised “transport” that they can then assault out of in the same turn, which can obviously be a game changer for Assault-focused unit like this.
Of all these, the top 5 psychic powers I would aim for in this battle group are Endurance, Invisibility, and Veil of Time for improved survivability, Magnetokinesis for improved mobility, and Prescience for improved close combat to Hit rolls.