Main damage comes from Ventana.  However buffing her by supporters isn't enough, therefore you have to use mercs who can use incoming DMG boost or stats reduction.  That said, Ogdoad has stats weakening immunity, so normal incoming DMG boost or stats reduction skills do not work.  So you have to use...

I have compared 15 combinations from Assault, Fatal, and Rage. Considering the best combos of low / high critical % or average. Avg Damage by Skill Levels +0 +4 +6 +9 [Shield 20.5%] + [Shield 20.5%] 4781 5481+15% 9334+81% 14062+99%   DMG Difference by HP% 25...

☆☆☆ KO Challenge / Turn Challenge ☆☆☆ ★6 Anais+9 ★6 Veronia+9 ★5 Octavia+5 (Kill 2, 3, 4, 5) ★6 Angelica+3 ★6 Grosa+9 ★6 Viola+9 ★6 Zakan+9 (Kill 6 by counter) ☆☆☆ No Supporter Challenge ☆☆☆ I couldn't kill 4 by Chalkle's counter, so I used Octavia to finish it up.  ...

Beating all challenges.  I had to upgrade Wiggle from ★5 to 6, because it was able to kill 3, and 4 by burn, but wasn't able to instantly kill 2.  Other important point here is Grosa's turn is right before Viola, so that even when Viola gets Concentrated Fire, Grosa is still the one who get attacked...

☆☆☆ KO Challenge / Turn Challenge ☆☆☆ 2, 3, 4, 5, 6 do good damage, so I am using Mercedes, Chalkle, Astrid's counter to counter them.  The boss and other mobs are taken care by Angelica, Viola, and Seir.  By calculation Astrid has to be +6 in order to kill mobs by counter damage.  Veronia also n...

Beating all challenges.  Use Zenith to take the boss's attack so that she can put Concentrated Fire on the boss.  Then Angelica will prevent the boss's heal and finish with Ventana+9. ★6 Angelica+3 (Kill boss) ★6 Ventana+9 (Kill boss) ★6 Viola+9 ★6 Seir+6 ★6 Rafina+9 ★6 Astrid+6 ...

Beating all challenges.  Use Zenith to take the boss's attack so that she can put Concentrated Fire on the boss.  Then Angelica will prevent the boss's heal and finish with Ventana+9. ★6 Noel+9 (Kill 5, 6, 7, 8, 9) ★6 Angelica+3 (Kill boss) ★6 Ventana+9 (Kill boss) ★6 Seir+6 ★6 Astr...

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.  ...

40-10 is mainly for leveling up 1 merc, the last 9th spot.  However you can still get normal scrolls, and good amount of golds.  Need 8 horseshoes for hard mode.  When other 8 mercs are max level, 9th merc will get 6245 exp (781 exp / horseshoe) which is better than 45-10. ★6 Arines+9 ★6 Ebo...

25-10 is a good place to get normal scrolls, and leveling up for several mercs.  Need 6 horseshoes for hard mode.  Sometimes Angelica fails to land critical, and won't be able to one-shot the boss.  This formation is tuned for clearing as fast as possible, but you can replace Zakan, Chalkle, and Eun...