As I have thought about the new Genestealer Cult codex and how to apply it competitively, one of the things that occurred to me is to try and maximize their access to Astra Militarum as Allies of Convenience. Thinking of this and looking at what the AM have access to, the best ally for a Genestealer Cult army that I can think of is the “Emperor’s Wrath Artillery Company” formation, for the following reasons:
 Ignore Cover Basilisks — With the ability to give Orders to artillery units in the formation, you can have up to 6 x S9 AP3 Large Blasts with Ignore Cover shooting each turn (and a 7th S9 AP3 Large Blast from the Master of Ordnance, if you take him in the Command Squad). This rectifies one of the weaknesses of the Genestealer Cult force, namely long-range high strength/low AP shooting and also provides them with very solid options against enemy infantry and light-medium vehicles at range. Put another way, Basilisk batteries from this formation provide excellent “fire base” support to an otherwise light-weight, assault focused, guerrilla tactics “horde” army that the Genestealer Cult forces specialize in.
 Multi-purpose Command Squad support — While they cannot directly help Genestealer Cult units, being only Allies of Convenience, the mandatory Command Squad from the “Emperor’s Wrath” formation provides multiple indirect aids to a Genestealer Cult army. First, if you take an Officer of the Fleet, he can use his ability to subtract -1 from enemy Reserve Rolls in conjunction with the -1 to enemy Reserve Rolls from the Genestealer Cult “Decurion” to cause enemy Reserves to come in on a 5+, which can be pivotal against opponents that keep a lot of their strength off the table during initial deployment. Second, the Astropath can cheaply add additional Warp Charges to your pool for use by the Patriarch and/or Magus and the Master of Ordnance, as mentioned above, can add a cheap additional S9 AP3 Large Blast to your fire support. Finally, besides issuing orders to the Artillery vehicles, the Command Squad can provide up to 9 x S7 AP2 Ignore Cover Plasma shots a turn to deal with MCs or 4 x S8 AP1 Melta shots a turn against vehicles, both of which capabilities are general lacking in the Genestealer Cult arsenal.
 “Nuclear Option” Deathstrike Missiles — While being a One Use Only weapon and not able to shoot until Turn 2 at the earliest, the Deathstrike Missile provide a S10 AP1 Ignore Cover shot with a 10″ Apocalyptic Blast that combines beautifully with a Genestealer Cult army by putting your opponent on the horns of a dilemma… if they concentrate on shooting at your mobile close combat units like Purestrain Genestealers, Metamorphs, or even Neophytes with supporting characters to buff their assault abilities, then that leaves the Deathstrike free to launch its very powerful attack (i.e. imagine hitting an enemy Thunderwolf Cavalry squad or Grav Centurion squad with the Deathstrike Missile… the S10 AP1 Ignore Cover doubles out their T5 and ignores everything but Invul saves)…. on the other hand, if your opponent focuses on destroying the Deathstrike to prevent if from firing, then that means their is less combat power to deal with the massed “Cult Ambushing” Genestealer Cult infantry that can then shoot/charge into combat on the following turn.
 Manticore, Hydras, and Wyverns — A Manticore provide additional S10 Ordnance pie plates at long range for dealing with both vehicles and infantry at an economical points cost, if you choose to take this instead of the Deathstrike, while Hydras provide air-defense/skimmer defense that Genestealer Cult armies otherwise have very limited capabilities in. Hydra squadrons combine especially well with the Ignore Cover orders in dealing with Skimmer heavy armies like Mech Eldar, Mech Tau, Mech Necrons, Ravenwing armies, or even Chaos Daemon “Flying Circuses” and enemy Tyranid “Monster Mash” lists, since they can put out a high volume of accurate S7 shooting that ignores enemy Jink saves. Finally, Wyverns provide incredible fire support for any force and are a threat to any enemy infantry, even TEQs, due to their accuracy, high volume of shooting with the Shred and Ignore Cover USRs, and cheap points costs (they provide a very strong supplement to Genestealer Cult forces when combined with the Pinning order, since they “suppress” enemy shooting units for a turn, thus buying the Genestealer Cult forces time to survive before getting into assault, where they belong).