Point 1：The key for Zaraton is Sabrina.  If she can survive the boss's attack and give Hunter's Will to the party twice, then you will get much higher score.  Therefore I recommend to upgrade her to 6 stars.

Point 2：The rest is basic.  Use mercs with high basic points.  For the recommend mercs, please refer here. If you are using Kaoli, you should try to get set effect by including Barbara and Ceres. Likewise you should test set effect for Granhildr and Aaron as well.

Point 3：For supporters, mercs who have very strong barrier / DMG reduction are useful.  Obviously mercs with healing skills are also recommended.  And don't forget increasing AGI can help reduce Zaraton's DoT attack duration.

Point 4：With this particular setup, For the front 3, I used mercs who have instant heals as well as heals over time, because they don't get support from Sabrina.

Note：Here I am not using the proper runes, so the score is lower than what it could be.

1. ★6 Benshina+14
2. ★6 Jayden+15
3. ★6 Grosa+10
4. ★6 Meliodas+1
5. ★6 Ceres+10 (Set effect: Kaoli)
6. ★6 Barbara+3 (Set effect: Kaoli)
7. ★6 Veronia+14
8. ★6 Aaron+0
9. ★6 Granhildr+15
10. ★6 Kaoli+12
11. ★6 Themis+9
12. ★6 Sabrina+3