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.  Thanks to Krull's endless pain, The debuff will be carried to next round. For this formation, on 2nd round, Noel will increase damage by incoming DMG Boost.  Then I give 4 buffs to Chalkle who has the highest damage among my mercs.  And apply 2 buffs to Niya and Angelica who's 2nd best damage dealers for this boss.

1. ★6 Arines+9
2. ★5 Hyeon Wol+9
3. ★6 Ebony+9
4. ★6 Noel+9
5. ★5 Melody+9
6. ★6 Veronia+9
7. ★6 Chalkle+9
8. ★4 Karg+9 (Status-70%)
9. ★6 Viola+9 (Status-35%)
10. ★5 Krull+6 (Endless Pain)
11. ★6 Niya+9
12. ★6 Angelica+3