I would let go of:
EXO: Sehun, Xiumin, Suho, Kai, (Chanyeol if 5 have to leave)
f(x): Amber, Victoria
2NE1: Minzy (lol), Dara
Red Velvet: Irene, Wendy, Yeri (if 3 have to leave)
SHINee: Onew, Key, (Minho if 3 have to leave)
BP: Jisoo, Lisa
Big Bang: Daesung, Taeyang, (T.O.P if 3 have to leave)
I would let go of:
EXO: Sehun, Xiumin, Suho, Kai, (Chanyeol if 5 have to leave)
f(x): Amber, Victoria
2NE1: Minzy (lol), Dara
Red Velvet: Irene, Wendy, Yeri (if 3 have to leave)
SHINee: Onew, Key, (Minho if 3 have to leave)
BP: Jisoo, Lisa
Big Bang: Daesung, Taeyang, (T.O.P if 3 have to leave)