☆☆☆ Without Eldora / Mamonir / Karin ☆☆☆

First thing you have to do is to use stats weakening skills. (Please see here for recommended mercs with weakening skill) 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. At last use mercs like Niya who have multiple attack skill to get high score. However, to be hornet, I don't think I can improve too much with this strategy.

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

☆☆☆ With Eldora / Mamonir / Karin☆☆☆

With this strategy, use mercs with incoming DMG boost debuff, and stack it as many as possible. Then attack with Eldora or/and Mamonir to do big damage. (Please see here for recommended mercs with incoming DMG boost skill) I have tried with ★5 Karin+3 with the same setup, and she scored 750k alone. I don't know how much more damage with +10, but if you don't have Eldora and Mamonir, and struggling, maybe inventing on her might be a viable option. I'm guessing Valze is another one, but I don't have her, so I can't confirm.
Update: I saw a Twitter saying scoring 2.5 million with Karin.

1. ★6 Ceres+10
2. ★6 Naressa+10
3. ★6 Mary+10
4. ★6 Martina+9 (Incoming DMG boost +75%)
5. ★5 Zarka+10 (Incoming DMG boost +145% / Status -65%)
6. ★5 Orienne+9 (Incoming DMG boost +100% / Status -50%)
7. ★6 Esther+9 (Incoming DMG boost +65% / Status -50%)
8. ★6 Charlotte+10 (Incoming DMG boost +35% / Status -80%)
9. ★6 Gloria+2 (Incoming DMG boost +35%)
10. ★6 Denarisa+10 (Incoming DMG boost +125%)
11. ★6 Veronia+10
12. ★6 Mamonir+2