Sen no Kiseki II - Last Boss Nightmare No Damage

Pretty easy if we abuse Chrono Burst and Grall Burst Lost Arts. Just make sure Chrono Burst user have enough speed stat so he/she can get turn before the boss, and set Grall Burst user SPD a bit below Chrono Burst user, so he/she can get turn after everyone except Grall Burst user do S-Craft. Check video for more detailed