First half, use mercs with decomposition skill, then use Endless Pain to extend duration.   One thing to keep in mind is that Ogdoad's AGI is 100% and that means debuff duration is cut down to half.  Second half, apply Concentrated Fire as many times as you can , then finish with Ventana.

1. ★6 Arines+9
2. ★5 Kozak+9 (20k)
3. ★6 Ebony+9
4. ★5 Deomaron+0 (11k)
5. ★4 Joseph+5 (Endless Pain)
6. ★6 Niya+9 (13k)
7. ★5 Viola+9 (19k)
8. ★5 Krull+6 (Endless Pain)
9. ★4 Natalie+9 (Concentrated Fire)
10. ★4 Maya+0 (Concentrated Fire)
11. ★6 Veronia+7
12. ★6 Ventana+6 (63k)