How to beat last boss in regalia of men and monarchs?

I am playing on normal mode, so I cannot skip battles, I finally managed to get to the end of the game but the last boss is brutal.

10k+ health and he gets 1k of shields per turn (at first one can take shields off by killing his soldiers). Apparently, there is a hack with Harieta to move the instadeath debuff to the boss, but I didn't pick her in diplomacy quests. The other suggestion to use Griffith as a punching bag does not work, since boss resists silencing...

What characters and strategy could I use to beat the last boss?

Ok, so I figured out a way by using one of 'specially acquirable' characters Esther (If you don't have her sorry).

Characters: Esther and Diego essential, then Signy and some dps (Theo,...)

The setup before battle: Give Esther perk Synergistics so that she would AOE Energised

It's good if Signy has No stone unturned(5) also if Esther has Hollow-Point Blast(2) perks, but not essential.

Strategy:
0. Kill all the soldiers don't touch Carran he will parry, once they are down, move to 1-4
1. Get Esther to Energise Diego and Signy
2. Make energized Diego cast Target dummy next to Carran (no cooldown for energized).
3. When you have 2 authority points (also energized) Singy cast Massacre (~2k dmg)
4. On turns that target dummy is next to Carran Esther can shoot Penetration Round (~1k)

This way Carran should be deadlocked by Target dummy till he's dead. He will still be casting shield though, but you should have enough DPS...