If Octavia (spec'd on critical) can land critical hits on all 5 slime, 6 turns is possible.  Roll few times.

1. ★6 Mora+8 (Kill 2)
2. ★5 Octavia+3 (Kill 3, 4, 5, 7, 8)
3. ★6 Leto+9 (Kill 6)
4. ★5 Zakan+6＆★5 Jin+1 (Kill 1 by counter. If you have stronger Zakan, you don't need Jin)