By the first boss I assume the fight with Therese and Marsha. Well. Therese does have a Last Guardian skill that permanently give her 80% damage reduction buff. But, if you notice, Davina's first skill, Dispel Strike, can remove the target's buff. By using this skill, you can remove Therese's tankyness easily. Also, this is the first boss. Of course the player doesn't need to be in any danger when fighting the first boss.
Anyway, thank you for trying my game.