First things First make sure that you have got NC and unity buffs from feast and candy if you have it.

Enter the raid and do all 5 upgrades and leave

Enter again and kill all 4 gates in the 2nd room then kill the boss in the 1st room as quick as possible

Go to the 2nd room and make sure u stand in the area before putting blessing up

Then go to the corner in the little 3rd room like so

Apply Blessing in the corner then next turn move to the attack square to attack boss then move back to the corner again to apply blessing again, rinse and repeat until boss is dead and collect your reward.