Ok.. whoever gets the chance finish up 16,17, get as most hits in as you can, before finishing them.
Alice O8, attack 16
Tenshi N8, attack 17
Then, Kogasa/Youmu and Rumia finishes either 17 or 16 (Depends on if Kogasa can move after attack, she can attack one of those 2 targets, if not, she will just move to O9). And Actually who you want to give EXP more to? Kogasa or Youmu?
Other than that, move out on forts, and take cover, while getting the most armoured units up the front, and others behind. First heal up, Byakuren heal on Youmu/Alice, while our fast attack, Sanae, will take the fort on 13L.
Otherwise, stay put one more turn, use it for healing, and move out after everyone is fully healed.