Divine Guidance(the AoF that turns a 6 to wound into AP1) very much relies on weight of numbers. Roughly 1 in 6 hits will be turned into an AP1 wound (though you'll often feel the dice are against you), so don't bother using it unless you just caused a lot of hits or feel very lucky. You make the test after you know how many hits you get, so this is relatively easy
Large squads and Rapid Fire is key. Ignoring the Melta for a moment, in RF range your 15 Sister squad pumps out 25 S4 shots and an S5 template, which at BS4 means you can expect about 17 S4 hits (and hopefully a couple S5 ones) which means Divine Guidance should result in about three 6s to Wound. It's best used against smaller squads of expensive units, and it's a good way to make sure the few wounds you get on high-Toughness enemies COUNT when you run out of options (for instance, a Carnifex that's about to use your squad for a chew toy. And yes, it's still much preferable to off the thing with an Exorcist at safe distance).
Okay, comments on army...
First Canoness, if you intend to run her alone a Rosarius might be a good idea... admittedly, at that point you might as well go with Celestine instead as she's getting uncomfortably close. If she's supposed to be flying with the Seraphim, she should probably break off at some point so the Seraphim can utilize their Hit and Run.
Second Canoness, the Holy Icon on the Rhino is
a waste of points if it sticks with her. Mostly because it increases LD to a maximum of 10 and the Canoness already HAS that. For the same reason(Canoness has Ld10, and a book), you could shave the Book off the Retinue's VSS... unless you plan on martyring the Canoness, but I think she's too expensive for that as she currently is
For the Celestians, I'd advise AGAINST using Hand of the Emperor (the AoF with +2S) a lot of the time, actually... Note the Celestians have I4, which will often have you going first, and it combines nicely with their Holy Hatred rule to cause a lot of early hits. Unless you're assaulting something that won't really fight back/cause a lot of casualties, trading it for Initiative 1 (and going last) will likely hurt you more than him. Whether the +2S is worth the casualties does somewhat depend on the enemies' Initiative, Toughness and Saves, admittedly, but losing (the attacks of) a couple Sisters means you're causing less Hits and likely to cause less Wounds in the end. Not to mention you'll RECEIVE less if you manage to knock off a couple of them first.
Given that you're expecting these to get into CC, give the Meltagun to another squad and pick up another Flamer or a Storm Bolter. Perhaps get Frags and/or add a Sacred Standard as well?
Skipping to the Seraphim, have you considered the idea of dropping the Eviscerator and giving all these ladies some nice Melta Bombs to play with? Given the sheer amount of shots and CC attacks they give you(and the bit where the VSS suddenly contributes more to those), I really don't think you'll miss the two Evicerator attacks THAT much...