Looking at the new Cult Mechanicus book, one of the units which pops out at me immediately is the Kataphron Destroyers, which are a Troops choice for these scions of Mars.
Kataphron Destroyers are S/T5, so pretty durable despite their 4+ armor save, and they have excellent firepower in the form of either 24″ Heavy 2 Plasma Cannons or Heavy Grav Cannons (30″ Salvo 4/6 Grav weapons). They can move and shoot at full capacity due to special rules granting the equivelant of Relentless and they make up for their BS3 with the high volume of shots and great range of theur Heavy Grav-Cannons. What’s more, you can take up to 12 of them in a single squad and they only cost 2 PFs and a MB each, making them very affordable compared to Grav Cannon Centurions. Against anything with a 4+ armor save or better, they should be absolutely devastating.
Right out the gate, I think one of the best ways to run Kataphron Destroyers is to use an allied Space Marine Mantis Warrior force with Ahazra Redth as the HQ (and as your Warlord). Redth is a PML(2) Librarian with one guaranteed power (Mirage — gives Redth and his unit Shrouded and makes any unit charging them have Disordered Charge) and access to Divination, Pyromancy, Biomancy, and Telepathy for his other powers (one roll on a chart, plus Primaris of that power). Additionally, he has a Relic that allows him to re-roll his first failed Psychic Test and, importantly, his WL Trait grants him and his unit both Interceptor and Night Vision USR.
So, how does he work with Kataphron Destroyers? Attach Redth to a squad of 12 Destroyers, then place them in the center of your DZ (i.e. with maximum coverage/LOS around the board) behind an Aegis Line.
The ADL gives everyone a 4+ cover save, which Redth can then increase to a 2+ by casting Mirage (for Shrouded USR), thus mitigating the Destroyer’s mediocre armor save. He can also automatically get Prescience from the Divination discipline, so if he succesfully casts that on the Kataphron Destroyers he increases their shooting accuracy from 50% to 75%, which massively buffs their shooting potential.
Finally, since he gives the whole unit Interceptor with his Warlord Trait and the Destroyers have a 30″ range for their Heavy Grav Cannons, this battle group can provide an exceptional, 60″ bubble of counter-fire to deep-striking enemy threats, such as DPs with suicide melta units, Eldar Falcon squadrons, Dark Eldar Webway Portal jumps, Tau Crises Team drops, Gates of Infinity shenanigans, Tyranid Mawlocs, Asssault Terminators, Veil of Darkness Necrons, etc., etc.
With Prescience cast, 12 Kataphron Destroyers put out 72 shots a turn, resulting in an average of 54 hits.
Against a squad of 10 DSing Hammernators with 2+/3++, this yields an average of 52-53 wounds, which in turns causes 17-18 unsaved wounds, wiping the entire squad out almost twice over.
Against Farsight and a squad of 7 Crises Bodyguards with 3+/4++ saves, this yields an average of 48 wounds, which in turns causes about 24 unsaved wounds, again wiping the entire squad out entirely.
Against a squad of 10 Veil of Darkness Lychguard with 3+/3++ and 4+ RPs from the Decurion rules, this yields an average of 48 wounds, which in turns causes 17-16 unsaved wounds after 3++ saves, and finally 8 wounds after Reanimation Protocals, killing all but 2 Lychguard as they arrive.
Against a squadron of 3 Falcons DSing with no Scatter, this yields 9 HP/Immobilized Results, destroying the squadron completing before it can even shoot ot diembark any troops.
Against a unit of 10 Wraithguard with D-Scythes using a DE WWP to arrive with no scatter, this results in an average of 48 wounds, meaning that not a single Scythe-Guard even gets a chance to shoot its D-Weapon.
Against a deeps-triking Mawloc or Trygon, this results in an average of 48 wounds, killing the MC handily before it gets any opportunity to get stuck in or do any damage.
As you can see from the above Mathhammer, Redth with a maxed out squad of Kataphron Destroyers makes for an incredible mid-field firebase that can dominate the board and provide amazing protection against enemy DSing threats. Even better, with their T5 and potential 2+ cover saves from ADL + Shrouded, the Destroyers can absorb a lot of enemy firepower themselves and keep fighting. The critical vulnerability of this battle group is, of course, close combat, so you need to shield it from enemy assault units, but with Prescience cast and 72 shots, that means on Overwatch the unit is likely getting 20+ Grav Cannon hits on Overwatch (the Destroyers are not Slow and Purposeful and so CAN overwatch, unlike Grav Cannon Centurions), which should tear up any charging unit with and armor save of 4+ or better.