1. Level Aya's Raven Venom to level 2 (15 hits) or level 3 (22 hits). Level 1 is only 7 hits and will probably not be enough.
2. Take the recommended party: Capell, Aya, Vic (Butterfly Rush), Komachi (Thistle)
3. Go to the Bihar Mines floor in the Seraphic Gate. If you didn't know, you could just walk down without fighting.
4. Unequip weapons so you don't kill the mimic too fast. Set combat tactics to Focus.
5. Assign Raven Venom as Aya's connect skill. Connect to Aya.
6. Aggro the mimic, and back up to dodge breath attack. Hit Raven Venom as you do this (3.25 second cast time)
7. When breath attack is over, walk up and hit A,A,B (diminuendo dive). If you timed it right, the Raven Venom attack should land around the same time. You just need a handful of hits between you and the other two members to bring the combo to 20.
8. If you fail, sheath your weapon and try again.
I kept getting streaks of 17,18,19. As soon as I leveled Raven Venom to level 2, I got a 27 hit streak within 3 tries.