Main damage comes from Ventana.  However buffing her by supporters isn't enough, therefore you have to use mercs who can use incoming DMG boost or stats reduction.  That said, Ogdoad has stats weakening immunity, so normal incoming DMG boost or stats reduction skills do not work.  So you have to use skills which have incoming DMG boost or stats reduction as secondary effect.  For example, Noel's Burn has incoming DMG boost effect.

1. ★4 Alicia+9 (Incoming DMG boost)
2. ★6 Arines+9
3. ★5 Hyeon Wol+9
4. ★6 Ebony+9
5. ★5 Deomaron+0
6. ★6 Noel+9 (Incoming DMG boost)
7. ★4 Diane+9 (Incoming DMG boost)
8. ★4 Natalie+9 (Concentrated Fire + Stats reduction)
9. ★4 Maya+5 (Concentrated Fire + Stats reduction)
10. ★5 Krull+6 (Endless Pain)
11. ★6 Veronia+9
12. ★6 Ventana+9