One of the stronger synergistic combinations using Astra Militarum and Sisters of Battle is as follows:
-Astra Militarum “Steel Host” Formation (Leman Russ Command Squadron, 3 x Leman Russ Squadrons, 1 x Hydra). Besides all the heavy armor this formation brings to the fight (which is critical when combining with a SoB force), all models in this formation also gain Preferred Enemy as long as they are within 12″ of the Command Tank.
-Squadron of three Penitent Engines, costing just about the same price as a Land Raider total. They are fragile and take up a HS slot, but have outstanding damage output (6 x Heavy Flamer Templates followed by 18 x S10 AP2 attacks striking at Initiative 3 on the charge).
So, how do these two units work together? Well, in term survivability, the Steel Host Formation can be placed in front, using its AV14 and high threat value to absorb attention away from the Penitent Engines (who can also gain a cover save by staying obscured by the Leman Russes). Each turn the Steel Host moves up 6″ and pounds the enemy with formidable firepower (you have to take at least 5 Leman Russ variants, all of which can target different enemy units because of Tank Orders and they fact they are in different squadrons) while th Peninent Engines move up behind them. If an enemy unit moves up to assault the tanks (which is one of their primary weaknesses), then the Penitent Engines run forward and “counter-assault” the enemy close combat troops. With their tremendous strength against both hordes (i.e. 6 Heavy Flamer Templates, plus close combat attacks) and elite infantry (i.e. 18 x S10 AP2 attacks at initiative shoul dmake most TEQ shake in their boots), they can reliably protect the Leman Russes from close combat threats. Also, if the enemy does not pay attention to the Penitent Engines as the whole battle group advances, then eventually (i.e. by turn 3-4) they should be able to get into assault in the enemy DZ, since they are moving up 6″ every turn.
Either way, these two groups of units are quite good at mutually supporting one another and covering their respective weaknesses, thus forcing your opponent in to a dilemma on how to deal with them. Finally, you can flesh out this entire army with more SoB troops from a CAD and use the Steel Host formation’s mandatory Hydra to provide the SoB Primary Detachment with much needed anti-flyer/skimmer ability (e.g. put the Hydra on a Skyshield Landing Pad along with a pair of Exorcists to provide a durable, long-range fire support base for the entire army).