Boss Guide: Ch12-5 (Gotterdammerung)

Additional Guides

Enemy Layout

Drops	Saber Monument x1
Rewards	Saint Quartz x1 Aurora Steel x3

	Napoleon (Must choose one of two Guest Supports) Lv80 NP4 10/10/8
Sideline Support		Allfather Seal, Mold Excitation: Used at Battle Start by Brynhild when Napoleon is selected as Support. Remove Sigurd's [Dragon Black Spot] Passive. Decrease Critical Rate of all enemies by 20% [Permanent, Unremovable]. Increase Critical Star Drop Rate of all allies by 100% [Permanent, Unremovable].
When Backline		Artillery Support: Deal 10,000 damage to all enemies at the end of each turn.

	Brynhild (Must choose one of two Guest Supports) Lv80 NP4 10/10/8
Sideline Support		Artillery Support: Deal 10,000 damage to all enemies at the end of each turn.

Sigurd
	HP	237,575
	LVL	90
	HP	265,525
	LVL	90
	HP	307,450
	LVL	90
	Class	Saber
	NP Bar
	NP Type	ST
	Actions	Up to 3 per turn
	Traits	Humanoid, Servant, Male, Brynhild's Beloved, Divine, Dragon, Heaven or Earth, Riding, Weak to Enuma Elish

Battle Start 1		Dragon Black Spot: Increase Critical Strength by 50% [Permanent, Unremovable]. Will be removed if Napoleon is brought as Support.
Battle Start 2		Mental Debuff Immune: Apply Mental Debuff Immune [Permanent, Unremovable]
Battle Start 3		Flame Blade: Inflict Burn (400 dmg/turn, 5 turns) when attacking with normal attacks [Permanent, Unremovable].
Break		Command Spell: "I order you with my Command Spell... Smash them.": Increase Sigurd's Critical Chance by 100% (3 times, 3 turns).
Break		Command Spell: "I order you with my Command Spell... Rain down destruction.": Increase Sigurd's NP Gauge to MAX.
Skill 1		Primordial Rune (Warrior) B: Increase own Critical Strength by 100% (3 times, 3 turns).
Skill 2		Dragon Modification EX: Increase own Buster Card effectiveness by 50% (1 turn). Apply Guts to self (1 time, 3 turns). (Revive with 1000 HP)
Skill 3		Crystallized Wisdom A: Increase Critical Rate for one ally (1 turn). Apply Debuff Immune to one ally (1 turn). *Unsure if used [Confirmed by Jeffglen]
NP		Bölverkr Gram: Deals damage to a single enemy. Deals extra Special Attack damage to [Dragon] enemies.

Sigurd is a hefty boss with three HP bars. At the start of the fight he will apply a 50% Critical damage up buff, Mental Debuff immunity (no charm, fear, or terror), and a 400 damage per turn stackable burn. This burn can be cleansed. He comes equipped with skills to double his critical rate and to increase his Buster attacks.
Ophelia will support Sigurd with her command spells, giving him 3 guaranteed critical hits over 3 turns on the first break, and a full NP gauge on his second break.
Masters can only choose the NPCs Brynhildr or Napoleon as support. Napoleon will deal 10,000 damage per turn if he's not in the front line, and remove Sigurd's 50% Critical damage up buff if he is.
Due to class advantage and the added benefit of removing Sigurd's Critical damage up buff, selecting support NPC Napoleon is recommended.
Sigurd can be quite challenging if he can stack burns up on a Master's team, or get off some of his painful Critical hits at inopportune moments.
Still, Sigurd has a multitude of traits (Dragon, Divine, Male, Riding) which can provide massive damage against him.

In general, a Master will want to take advantage of his traits and bring counters such as Euryale or Orion (Male), or Riding/Divine (Oda Nobunaga). With proper setups, specific servants can drain his bars one shot at a time.
Servants with taunt or teamwide invulnerability are nice additions for his guaranteed critical hits and Noble Phantasm turn.
In general, one is looking more to burst him down than outlast him, making any kind of damage support and Noble Phantasm charge much appreciated.
Burns can be cleansed if they get to be too much, so a debuff cleansing servant is always helpful.

Note that these videos are not affiliated with GamePress, but showcase some lineups that are effective in clearing the final challenge. These can serve as a good reference for how to rotate skills during this quest or simply to have a look at the encounter before diving in.