Point 1: The key for Zaraton is Sabrina.  If she can survive the boss's attack and give Captain's Encouragement to the party twice, then you will get much higher score.  Therefore I recommend to upgrading her.

Point 2: The rest is basic.  Use high HP mercs with vital runes.  For the recommend mercs, please refer here.

Point 3: Front 3 are outside of Sabrina's AoE. Thus merc with instant healing and heal over time, or Mamonir who has Energy Guard are recommended. Veronia can also protect #1 merc by increasing DEF to 100%.

1. ★6 Kaoli+13
2. ★6 Granhildr+15
3. ★6 Mamonir+15
4. ★6 Benshina+15
5. ★6 Baine+10
6. ★6 Grosa+10
7. ★6 Valtor+14
8. ★6 Sabrina+10 (Healing)
9. ★6 Lucius+2
10. ★6 Floria+12 (Increase HP + Healing)
11. ★6 Meliodas+0
12. ★6 Veronia+15 (DEF100% for 1)