First thing you have to do is to use stat weakening skills.  This is not only to increase the damage to Arcstar, but also when you attack Arcstar, it has Overcharge which is like a counter damage, therefore it also reduces the damage you take.  It's ok that mercs who apply stat weakening will die.  Just make sure other attackers won't attack before weakening.  If Karg acts before 8th, his curse will expire before Arcstar's turn, and Ceres will die, so be careful.  In 1st round, Karg's curse will weaken Arcstar, however it's AGI will only go down to 30%.  Therefore you might lose some score if partially hit happens.  In 2nd round, Seir and Angelica's counters will put curse to Arcstar.

1. ★6 Anais+9
2. ★6 Ceres+9
3. ★6 Vincent+9
4. ★6 Ebony+9
5. ★6 Noel+9
6. ★6 Seir+10 (Status-50%)
7. ★6 Chalkle+10
8. ★4 Karg+9 (Status-70%)
9. ★6 Wiggle+9
10. ★6 Angelica+6 (Status-65%)
11. ★6 Veronia+9
12. ★6 Niya+9