My first observations are on how they interact with some of the newer rules and then some more tactical observations.
Overwatch: One of their most glaring weaknesses is in melee. Simply put, the Sisters of Battle cannot hang with most moderate quality assault units. The Overwatch mechanic rewards the Sisters of Battle with a way to even the odds and there is little in the way of disincentives for taking flamers. Clearly no army boasts more flame than the Sisters of Battle. This is accentuated by another rule:
Pistols and Grenades! They have them now. Vehicles fall to Krak grenades just fine now that you’re hitting on 3+ at worst. Grenades have made it a no brainer to have a lot of flame and mobility, even over harder hitting options because grenades can do what meltas were often called to do. The base Sisters of Battle unit I used in all cases took just Flamers and Combi-flamers. The impressive damage on offense and on defense is fairly dramatic. Another rule that helps them make this choice is that they have PISTOLS now. This always seemed like such a necessary change to me and I am glad they did. Pistols make them much more deadly and a single Grenade can even be thrown by one of the sisters in place of their pistol shot before they charge if they even need to!
Challenges: As squishy as Sisters are, it was quite common for an enemy IC to detach and attack a unit of Sisters on his own. With the rules allowing you to re-roll dice in a challenge once per 5 chix in the unit, this REALLY makes up for the strength and stat differences between those charging and the leader being charged. Challenges insulate the rest of the unit from overkill as an added bonus. I literally can’t count how many times opponents “dissed” my Sisters by attacking with an IC. They were right to do it. Now my IC’s and my Sister Superiors are at a more even footing, and could even kill an aggressive IC with a bit of luck. They can take Power Axes too, which enhances the Sisters of Battle in attempting this feat; and a 6+ invul tilts the wheel yet further. So while they will never be overwhelming favorites in challenges, just the protection of the unit from overkill will be worth its weight in gold many times.
So using the rule, the Sisters can either fire + absorb a charge without the challenge (to ensure they break and leave the enemy out in the open) OR they can fire pistols + flamers, charge and challenge, giving them a good chance of not ending up in the open themselves.
Mixed AP: you choose the order that wounds with different AP are taken. As such, mixed weapons units are now fairly interesting. Two of my Retributors have two Heavy Bolters and two Multi-meltas each. If a unit is being led by a Terminator sort, I can say “AP first”. If the Terminator is behind his men like the coward he is, then you just reverse it. Wound allocation has changed and had a profound effect on how useful or even “viable” (as some internet snoots are wont to term things) mixed weapon units are. Devastator Squads with a couple Plasma Cannons are a great example of how the Mixed AP can really be used to advantage with, say, Heavy Bolters for killing out of sight targets that you normally can't with Heavy Bolters.
Pre-Measuring: Staying at a maximum distance always had it’s virtue but now is easier. The force is more surgical because it knows EXACTLY how long it will take to execute the plan, when one can sacrifice movement, and when one can’t. Those pieces of information help
So then how to PLAY with this army now?
Pushing the field and getting right on top of the enemy aggressively seems an unlikely but excellent way to go. Meltaguns in your squads aren’t the absolute necessity they might have once seemed for the normal Troop Choices. Only Land Raiders and Flyers are really beyond the normal scope of the army to deal with thanks to grenades.
Gone are the days of trying to keep the game close until the end, then obliterating large chunks of the enemy with little in the way of return enemy fire. Though that was pretty wise before (as assault armies were just ridiculously awesome at killing us), it now is not as necessary. Wholesale slaughter as you go is A-OK and losses along the way are easier…much easier… to bear as I will explain. But first a word about deploying:
The Bell Formation works really well for surging forward. Rommel used it in
The force can overwhelm enemies with firepower that is both intense and affordable at short range. The problem you find is that the platforms they are fired from (Sisters of Battle) are super brittle and enemy’s will pick off minimum sized units with relative ease and will very often outdistance you enough to do it to key units before you do it to them. That leads to my next point.
While in other armies, minimizing unit sizes to allow maximum special weapons is popular, you might want to rethink that with Sisters of Battle. 10 man units, or at least LARGER units than are required, pay dividends for an army that MUST trek forward to be effective. For example, I use an 8 Sister Retributor Squad with 4 x heavy Flamers in a rhino to keep them viable longer and frustrate the enemy. Sisters of
On that course of thought, my Dominion Squads are fully 10 strong, which many have discouraged, but I encourage. The Dominions are simply devastating and if you use other units to screen while they do their grisly work, they can do it more than enough times to please any General. Similarly, the precious scoring units already start 10 strong. Embrace that not as a “tax” but instead as a smart move. In fact I am sporting a 17 Sister strong scoring unit to envelope my HQ. Warlord points are too important to just give up. In the Relic Mission, you can tie as long as you don’t lose the Warlord and do get First blood, something Sisters of Battle are rather adept at getting! If you go minimal in unit size, you might give up First Blood.
Seraphim need an Eviscerator if you take them. Definite must. I’d take one in any Seraphim squad and keep the squad small. It’s my exception to the "super-size" rule because they MUST deep strike so close to the enemy to be effective that there’s nothing you can do to mitigate charges. They kind of act on an island. All you can do is take a few with you on the way out. Still, as a “Drop wall” barricade, they work admirably and the pain they inflict is worthwhile. Waltzing them across a field seems a poor move though. They “outrun their coverage”, to use football parlance and become too easy as a target of opportunity.
Another Tactical point to be made: range is an issue for Sisters of Battle and putting your retributors on the far wings is a mistake. They simply can’t hit enough of the board when split off like that. One is hitting or the other is hitting but rarely both. So when deploying Retributors, they actually belong closer to center. With so many units screening for them, they are far safer and will fire more times per game in the end. This positioning also makes them harder to see by as many enemy units when obscured by the intervening vehicles, terrain and models. As the board thins out, they will have nice shots to take as your last line of offense (and defense at times). Cover saves aren’t AS big a deal now, so whereas giving enemies cover might have been a reason not to centrally deploy before, I’d say go ahead and place them closer to center now and even walk them up round 1 to get them into a better prepared position. Last point on those: being closer to center steals the “cheap kill” certain jump units or scouts are designed to get.
Don’t spend points on a Bastion to keep Retributors firing. The static position of the bastion and the Retributor range make this kind of an iffy proposition for the price. Sky shields are equally wasted. You just have no unit that can take advantage of it for the price.
Aegis Defense lines can be used though and set straight in the middle of the board to slow enemies down from one flank or the other. This little tactics of using it for that purpose rather than for static firing position plays to the Dominions strength as scouting units. If you put it in a “maze” formation, you can really increase the DEPTH of the terrain feature as well which hurts enemy movement for a longer period potentially. Having it farther forward is nothing but an asset for Sisters players, as Dominion take control of the cannon with their “off” models and patiently fire from behind it with tremendous effect. Just slowing an enemy’s move and/or charge for one round is big big big for Sisters of Battle who aren’t assault friendly. Slowing the enemy also feeds right into the Celestian notes (see below) as far as helping them do their job.
Battle Conclaves, Penitent Engines and Sisters Repentia are the legit melee threats in this army. Amongst them, the Sisters Repentia seems the most economical for the destruction they cause and a unit of 7 or 8 dishes out a lot of pain against all unit types. Now that their rules essentially are all positives and no negatives, you can REALLY use them to truck up as a counterattack unit. They can't be in a Rhino to start but are fleet and so can even afford to start in the open behind your wall of other stuff. The threat of Dominions and Retributors will likely keep the enemy focus on them plenty long enough to get up there and once there, the enemy now has multiple reasons not to want to charge anyways! For whatever unit charges first will likely eat Repentia chain fists. The Repentia ability to kill even in death is potent and not to be underestimated. I have simultaneously killed Terminator Squads with them even though the Terminators killed all of them! Pretty cool. Tactically, the Repentias need to lag behind in the early going so they don’t become a target too early. It is NOT a terrible idea to invest more points in the squad itself instead of a Rhino to carry them. The extra casualties they can suffer effectively
Celestians got a raw deal… Again…. Until you realize what they are best at: Charging the enemy and clogging them for exactly two assault phases at very low cost. If you foresee that the enemy will likely get the jump on you, feed them a garbage unit like a Rhino and then use the Celestians to defend the rest of the units for two phases by charging into melee Fearlessly, giving everyone behind them an entire round to kill other targets followed by setting the clogged unit up for annihilation as well. “Super units” sometimes just cannot be stopped any other way but to delay them and then hit them with everything, but committing everything before the rest of the army is softened can be tough. So Celestians are terrific at their cost for this duty when they go Fearless (either through their Act of Faith or through the bubble Kyrinov can provide). The stumbling block they represent is the other unit I would not max out in size, but 6 or 7 of them is about right against most foes.
A lot of this advice comes down to the concept of “Acceptable losses”. Your Sisters of Battle are scoring units until the last one dies! Objectives are more important in all missions than actual kills. 3:1!!! Stay at a 2:1 kill ratio and they cannot catch up if you take 3 point objectives, even when you seem to be losing the fight! Most armies will not have the personnel to keep taking losses like you can. Objectives WILL eventually open up from the bleeding. That’s the moment of victory for Sisters of Battle.
All those faithful allowed to die are vindicated and proven worthy by the victory they bring the Emperor and his loving servants!