Because I can't be paranoid enough:
"What did you mean, when you said that the Sendai girls aren't totally blameless?" you ask.
Homura doesn't respond immediately. "You should have asked before you left," she says tonelessly. "They were... not gentle about their... operations."
It depends on what she means by 'operations'. Presumably it corresponds to 'means of acquiring grief seeds', of which the primary means has been payment for healing (at least in the recent past, since Rin is a relatively new member). Other operations may have been in play prior to Rin's joining (possibly another healer that witched out after the lichbomb? a friend of one of the university group, the one that went to Tome?).
How could they 'not be gentle' about such matters? Aside from forcibly taking grief seeds....
We know that when the Tohoku group approached Mami, they were nervous, possibly scared of being caught. Likely paranoia about the main Sendai group finding out?
How likely is it that the Sendai group engaged in operations similar to the Tokyo 6 — using hostages and turning MGs into grief seeds?
Would they think that, rather than our grief control being a benefit, it's actually a threat? Their extortionate behavior is part of what gave them power. If we kill the value of grief seeds, their power tanks with it. Would they want to eliminate that threat that would undercut their power, despite what it would cost them? Unfortunately, humans are prone to such irrational actions...
Right, how would the Sendai set fight, as a group?
Water controller
Teleporter
Healer
Gravity control
Force fields
Gravity control to hinder mobility. Force fields for defense and to remove escape vectors. Teleporter for fast attacks that are difficult to defend against (and we already know that Sakura is quick on the lethal attacks). Water control and force fields can help counter most powers' direct attacks. Water control can also play subtle, trickles of water on the street that you don't notice til it's too late. Healing can probably be reversed, for nasty body decay attacks, if she gets into melee range.
They know of Sabrina's powers, and they should know Mami by reputation. They know nothing about Kirika.
If they attack with just the three of them here:
If they decided that we were more trouble than we're worth, they'd most likely focus on Sabrina first, as the greatest known threat. Akiko distracts us with a direct attack, and Sakura teleports for a fast kick or sword strike to our neck. Rin moves in to disable Mami, and then the three can turn to the unknown threat to deal with Kirika. They will likely be surprised by dullahan!Sabrina getting back up to attack, and Kirika's null blast should make this end pretty quick after that, as long as she has enough time to act.
They -might- attack Kirika first, as an unknown threat, but I can't imagine them being willing to let Sabrina act when they know what she can do with the grief in your soul gem.
If they get the entire group of 5 back together before attacking:
The first attack would likely be to separate us with force fields, and gravity attack to keep us stuck, followed by the above combo. While that would normally be effective in further reducing our options, Mami's guns and our grief manipulation should be relatively free even with the gravity attack, and the extra delay before they start the combo will allow Kirika to NOPE them before it gets off the ground. After that, it's slice and dice time.
It would be nice to ensure that such an attack never gets off the ground in the first place. If they plan to attack, they're likely to request actions that would lower our defenses first (eg: Can you put those balls of grief away? They're rather disturbing.) Perhaps a lining of grief inside our cloak, as a 'shield' against an attack from behind. Keep our senses tuned for any of them drawing on their magic. A keyword to get Kirika to null-magic (since it takes a few seconds to charge up)?
