If Rumia is going to confront the mage, she'll need hp. I'll think about what you all can do and post it later.
Working off the plan from IRC, I see this as the best-case scenario:
We get in position.
Rumia attacks 5. Rumia HP 12, enemy HP 5.
EP
5 attacks me. My HP 12.
6 attacks Meiling. Meiling HP 10, enemy HP 7.
7 attacks me. My HP 7, enemy HP 15.
8 can't do shit.
PP
Byakuren heals me. Back to full health, I think.
Meiling retreats and drinks. HP 20.
Cirno attacks 6 from range.
Youmu moves up and kills 6.
I and Rumia do absolutely nothing.
EP
5 attacks one of our front line units. (It could also attack Cirno, but then it would have an embarrassing death.)
7 attacks Youmu. Enemy has 11 HP remaining.
8 attacks me. Enemy has 14 HP remaining.
PP
Cirno gets the hell out of the way.
Rumia kills 7.
I kill 5, provided Cirno didn't already do that.
Meiling attacks 8. Meiling HP 10, enemy HP 2.
Youmu kills 8.
Healing.
Of course, as I said earlier, this is a best-case scenario. People could miss, then we'd have to fall back and/or take deaths. Also, figuring all this out was a massive pain in the ass.
> Sanae to K4, equip sword