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.  After it is weakened, apply Concentrated Fire as many times as you can , then finish with Ventana.  If your mercs are too weak to survive damage from Overcharge, you need to give immunity bestowal to protect them.  Once its ATK becomes low enough, you don't need immunity.

1. ★6 Anais+9
2. ★4 Karg+9 (Status-70%)
3. ★5 Viola+9 (Status-35%)
4. ★6 Arines+9
5. ★6 Mora+9 (28k)
6. ★5 Krull+6 (Endless Pain)
7. ★6 Ebony+9
8. ★6 Niya+9 (62k)
9. ★4 Natalie+9 (Concentrated Fire)
10. ★4 Maya+0 (Concentrated Fire)
11. ★6 Veronia+7
12. ★6 Ventana+6 (125k)