Sakurai the Cursed
Character should be at least level 40 before playing quests in Book 3.
That's a fairly strange notion considering new players can choose to start the game in Book 3 if they like, and finishing the entirety of the storyline probably won't even get you much farther than level 40 without outside farming, let alone before you start Book 3 at all.
Blue's quirk is that each time he casts the reflective shield (signified by the BPD buff and the popup: "Blue casts a powerful spell! His defense increases as his spell settles."), he'll inflict a hit of 100% for the first hit he takes, then 50% for the next, then 25%, and so on. This is tracked for the full duration of the shield, so if you inflict 2 hits on the first turn it's up, then the first hit on the second turn will reflect 25% of his damage, meaning the reflect becomes weaker and weaker as he takes hits until it's down to 0 and it's safe to attack him, even if the shield is still up. This count is also reset when he re-casts the shield, though, which he can potentially do every turn if you get unlucky as it has no cooldown. So the basic strategy is to kill Green first since he's the most dangerous and with the lowest health, then Red which will take longer but it's essentially 3 vs 1 so it shouldn't be too big a problem, and then when Blue is all alone you want to either not attack him at all when his shield is up (but this is very dependent on RNG, as he may choose to spam it), or at the very least simply don't use attacks with too many hits if the party member's HP is too low to take the reflect damage; remember that each party member has potions to heal, and/or to skip their turn if attacking would kill them. And if it comes down to it, you can bring a class with a focus on DoT damage, as that both ignores BPD and also does not trigger the reflect when it ticks; Necromancer comes to mind as a particularly great choice for this, since it has a passive DoT and a couple others, and multiple skills that don't hit the enemy as well to skip turns when necessary. This will of course be easier as you level and, more importantly, get better equipment (Doom and Destiny weapons are incredibly powerful for their level, so definitely do follow that advice and grab them), but it's doable at low levels, you just have to realize how he functions and work around it.