Point 1：Dagdarone will not take damage while it's in invincible mode. Only after it takes 18 normal attacks, it will take damage. So you have to use mercs with multi-attacks. With my setup, Aie is the one who blows 18th attack.

Point 2：Dagdarone will counter attack when being attacked while it is in invincible mode. You have to use supporter(s) to protect mercs. If you have Refithea, she's all you need for this task.

Point 3：Once you break invincible mode, not only Dagdarone take damage, but also debuff becomes effective too. Stack up strong Inc DMG boosts. Please see here for recommended mercs with incoming DMG boost skill here.

Point 4：For the last striker my suggestion is Barbara, but mine is weak, so I am using Mamonir. I believe any of mercs with the special skill that is effective against Arcstar is good here too. So that would be Valzé, or Eldora. Also Alicia, or Thalos are good too.

Point 5：This boss has a unique mechanic that every time it get attacked, it becomes Dagdarone's turn. So if in one round it get attacked 5 times, it also has 5 turns. This mechanic really does affects supporter's buff duration. Buff duration from old supporters are typically 10 to 12 turns which maybe too short. Make sure your attackes' buffs are not expired.

1. ★6 Refithea+15
2. ★6 Christina+15 (Multi-attack 5 x 2)
3. ★6 Wilhelmina+10 (Multi-attack 3. Need to die after attack)
4. ★6 Helena+10
5. ★6 Isabel+15
6. ★6 Aie+2 (Incoming DMG boost+200%)
7. ★6 Orienne+10 (Incoming DMG boost+125%)
8. ★6 Denarisa+10 (Incoming DMG boost+125%)
9. ★6 Zarka+10 (Incoming DMG boost+145%)
10. ★6 Lyudmila+13 (Incoming DMG boost+300%)
11. ★6 Esther+14 (Incoming DMG boost+100%)
12. ★6 Mamonir+6