I've taken some time to calm down and think this out, and I may have an idea. However, it's very very specific and relies on our order of movement.
This turn:
Cirno -> L10
Youmu -> L9
Meiling -> L7, trade for tea and drink it
Sanae -> L12
Rumia -> K8, DO NOT ATTACK.
Enemy phase - only the archer can reach us, she can't kill Rumia outright.
Next turn:
Byakuren heals Rumia
Rumia kills the archer
Sanae -> K7, trade for tea
Meiling, Cirno, and Youmu stay put
Enemies attack, our tanks get beat on
Next turn
Sanae retreats and drinks tea
Byakuren heals Meiling
The rest of you start killing people
So, um, is this plan good, or is it crap?