They rely on shock and awe
Overwhelming firepower, combined with some way to get all of them into the city quickly without the defenders realizing, for maximum surprise. Possibly a stealth type power, possibly teleportation. Magical girl powers are perfect for maneuver warfare, as we've shown in our own battles.
I suspect they have a clairvoyant of some kind, because another common thread is that they nearly always arrive in a perfect position to demoralize the defenders, often by destroying or occupying some location of significance, or even catching one of the defenders alone and then maiming them to send a message."
Locations of significance that I can think of:
Mami's apartment (just like in To The Stars)
School (especially since they can hold the civilians inside hostage)
Madoka's house (hold Madoka hostage, use a deadman switch so that Homu can't just instantly murderface them)
Homura's apartment (meh. It's an option, but I'm not sure if there's much point)
At least some of these options depend on just how much information they have on us. For example, Madoka is a major(albeit heavily guarded) weak point, but it's difficult to find that out without an in depth knowledge of how the group works. On the other hand, for all we know they may have that kind of knowledge if their clairvoyant is good enough, and more likely Kyubey may tell them in order to put Madoka in a situation where she's more likely to contract.
That being said, I think the obvious target is the school. Lots of hostages, they probably will have heard that we're a bleeding heart with regards to civilian casualties, plus even if they know almost nothing about us they would know that they have a good chance of getting a hostage that we know or have known personally. Not to mention they may very well get Madoka as part of the deal, since she goes to school too. An option for them if they have good information would be to send their main force to take the school, and send a smaller force (maybe one meguca, plus some disposable minions if they have those?) to take Madoka's house and try to get some of her family. Depending on how much they are able to spread their forces and how good their information is, they may also show up at Junko's workplace and take her hostage.
That only accounts for them attacking pre-Walpurgisnacht though. Attacking afterwards is somewhat of a gamble, as we should be greatly weakened from fighting the queen of witches, but at the same time we (Sabrina) may also be dead and/or have expended most of Mitakihara's resources as far as grief seeds and clear seeds go. Furthermore, if we do well enough we may have many allies left at the end who could be troublesome to fight. If they go this route, I would say that the most likely target would be the evacuation shelter where all the civilians go, making their move while we're in the end stages of the Walp-fight. Normally that should be an easy target, as it's a huge place with tons of people inside it and no magical girl presence. However, we've been planning to have Yuki use her magic on the evacuation shelter, which should allow her to instantly shift all of the civilians (especially 'Doka and family) to a different place at a moments notice. It might be too difficult for them to attack there, although if they have enough information they could try and send disposable minions (if they have them) to every place Yuki controls, preventing her from just dumping everyone in a safe place. This would be very difficult though, and requires them to have information on every single place that Yuki controls, be able to attack every place at once, and successfully occupy each one at the same time. Not easy, and not possible without tons of disposable minions (who are tough enough to not be immediately destroyed by Yuki), near perfect information on Yuki's powers/places, and a maneuverability power that lets them distribute the minions everywhere they need to go. As a better option, if they have someone with antimagic they could instead attack the place the civilians are at, and try to shut down Yuki's power before she can teleport the civilians out. This is much more feasible, but requires they have antimagic.
Another option if they attack post Walp-fight, is that they could grab and maim a couple defenders. This would be done by hit and runs attacks during the end stages of the fight itself, attacking meguca at the fringes, especially ones who are behind the lines in order to receive healing and use clear seeds. Probably a much easier option than attacking the civilians in this scenario.
"How many of them are there?" you ask. A frown grows on your own face. A clairvoyant. That does make things difficult.
"I'm uncertain," Yuki admits. "Accounts range from six to eleven."
While this doesn't fully eliminate the possibility of Iowa having or getting a minion-mancer, it makes it much less likely. I suspect that she who must not be named was originally supposed to escape and become Iowa's minion-mancer, but was allowed to be caught so that the thread could live. The more important question is whether Iowa will be getting a replacement or not.
"Hell yeah," Atsuko says. "I mean... honestly, I'd kinda like to match myself against a group like that. No holds barred."
I have to agree. This will be fun.
So it seems like the most likely targets are the school pre Walp-fight, and the weaker magical girls we bring in to help with Walpurgisnacht during and after the Walp-fight.
To deal with school as a target: Ideally it would be nice for Yuki to use her power on the school. If that's not feasible, I'm not sure what to do.
Madoka's family: Give everyone in Madoka's family one of Homura's shield enchantments. Maybe have Madoka give them to the family as good luck charms or something, and make them promise to always wear them. That would probably get them to wear them during the work day, which is really all we need since Homura wants Madoka's house to be under Yuki's magic so they should be safe while inside.
Walp-fight helpers: First off, we want as many enchantments as we can for everyone fighting (of course), and protective enchantments are one big thing we'll want. Not sure if it's feasible to equip everyone though, especially if Homura's the only one who can make them. Maybe have a fast response team that holds back and contains a lot of firepower and a maneuverability type (teleportation, timestop, etc)? They could plug gaps in our forces, and also would be able to respond to anything unexpected such as Iowa attacking our medics.
Mami's Apartment: Do they really want to drive both Mami Tomoe
and Sabrina Vee into a murderous rage, with them as the target? This problem is self-solving.
Edit:
This being magic, it's also fully possible that when she says shock and awe, she means they literally have someone who projects the emotions of shock and/or awe onto her opponents. Some kind of emotional manipulation magic, or a fear aura. Possibly someone like Cherie Vasil from Worm (sorry if I got her name wrong). To counter, I suggest something similar to the mind protection enchantments that we are developing for the Tokyo talk, or just blasting her from as far away as possible.