First thing you have to do is to use stats 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 stats weakening to die. Just make sure other attackers won’t attack before weakening Arcstar. 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.  It seems mages do not have enough buff, but actually buffs from 1st round are still effective, so don't worry.

1. ★6 Hyeon Wol+10
2. ★6 Melody+10 (Buff from 1st round)
3. ★6 Chalkle+10 (Buff from 1st round)
4. ★6 Seir+10 (Status-50%)
5. ★6 Arines+10
6. ★6 Ceres+10
7. ★6 Mary+10
8. ★4 Karg+9 (Status-70%)
9. ★6 Wiggle+10 (DMG boost+50%)
10. ★6 Angelica+6 (Status-65%)
11. ★6 Veronia+10
12. ★6 Niya+9