Boss: Megalos Ch11-5 (Agartha)

Enemy Layout

Fatal Battle 1/1

308,340

Megalos

Drops	Warhorse's Young Horn x1 Pt 3 Seed of Yggdrasil x1 Pt 3 Magic Gem of Lancer x1 Pt 3 Claw of Chaos x1 Pt 4 Seed of Yggdrasil x1 Pt 4 Magic Gem of Lancer x1 Pt 4 Phoenix Feather x1 Pt 5 Berserker Piece x1 Pt 6 Seed of Yggdrasil x1 Pt 7 Gem of Saber x1 Pt 7 Berserker Monument x1 Pt 7
Rewards	Saint Quartz x1 Phoenix Feather x3 (JP Only) Reveals the True Name of Rider of Resistance (Part 6) Reveals the True Name of Berserker of El Dorado (Part 7)

Megalos
	HP	308,340
	LVL	90
	Class	Berserker
	NP Bar
	NP Type	AoE
	Actions	Up to 4 (!!!) per turn
	Traits	Servant, Male, Humanoid, Divine, Greek Mythology Males, Super Giant, Weak to Enuma Elish

Active Buff		Revives with 50% HP (151,170 HP) upon death (12 times, Cannot be removed). After his 4th Guts is consumed, the battle ends.
Normal Attack		He has both a single target and AoE normal attack.
Skill 1		Increase own ATK (3 turns). Increase own Mental Debuff Resistance (3 turns).
Skill 2		Applies Target Focus to one enemy (1 time, 2 turns). Cast every ~3 turns (?) as the last action of the turn even if he is disabled. At the beginning of the next turn, he will use a special multi-hitting single target attack on the affected enemy. Note that if he is stunned or unable to act then he will delay its usage and attack someone at random.
Skill 3		Apply Evade to self (1 turn). Increase own Critical Strength (3 turns). Unconfirmed, we expect him not to have it.
NP Effect		Deals heavy damage to all enemies.

The fight begins immediately against Megalos, with the winning condition being to trigger 3 of Megalos’ Guts charges (i.e. “kill” Megalos 4 times). Each Guts charge restores him to half his maximum HP (154,170).
It’s possible to trigger multiple Guts charges in a single turn by taking advantage of how enemy “death” updates during a player’s turn, such as:

Going from Servant A’s regular card to Servant A’s NP after Servant A’s regular card depletes an enemy’s HP bar, and vice versa.
Going from any of Servant A’s cards to Servant B’s card after Servant A’s card depletes an enemy’s HP bar, and vice versa.

Unlike most Berserker bosses, Megalos has 3 actions per turn. One of his normal attacks can hit the entire party, similar to Raid Boss Ibaraki from Rashomon, though it is not a guaranteed crit.
Furthermore, on Turn 3, and every 3 turns after, Megalos will apply a Taunt debuff (1 hit, 2 turn duration) to one Servant at random in the party. This skill activates at the end of his turn, and will automatically be used even if Megalos is Charmed or Stunned. The turn duration counter updates at the end of the player’s turn, not Megalos’. Unsure if Skill Sealing prevents Taunt from being cast.
Megalos will unleash a massive ST basic attack the following turn, which can crit as well. This massive attack will still activate even if the Taunt debuff has expired or if a Master tried to stall him (the target will simple be random instead. Only Megalos's NP will have a higher priority.
All-in-all, Megalos is an intimidating opponent, but one that still has many weaknesses to take advantage of. The hardest part of the fight is burning through Megalos’ initial 308,340 HP while keeping defenses up or stalling against his highly damaging attacks.

Since Megalos is the only enemy to fight, single target NP Servants are a very good idea to bring, especially if they have some way to attract Critical Stars, have a strong NP battery skill, or can exploit bonus damage against one of Megalos’ traits.
Yet, don’t neglect bringing defensive or supportive NP Servants since Megalos can and will hit the party for an extreme amount of damage due to being a Berserker. A safe and generic setup would be something as follows:

Offensive single target NP Servant (With powerful offensive CE)
Mash/Support Caster (a CE with starting NP gauge, +X NP gauge/turn, improved NP Generation, Critical Stars/turn, and so on)
Support Caster (Same as above)
Georgios/Mash
Leonidas/Georgios
Offensive single target NP Servant/Cu/Cu Alter/Bond 10 Heracles (With Powerful Offensive CE)

This kind of setup provides a decent amount of toughness combined with a strong NP to use within the first few turns, while providing multiple ways of protecting the initial damage dealer in case the fight drags on.
In terms of selecting the right damage-dealer, Euryale is one of the most universal and powerful option to bring to this fight. She can stall with her Charms and drain his NP Gauge, as long as Masters make sure to start controlling Megalos immediately as he can buff his Mental Debuff Resistance. With a massively damaging Anti-Male NP, she can quickly chew through his health bars even with minimal offensive support.
There are plenty of other offensive options to consider: Anti-Male works very well, Anti-Divine work great, Berserker of El Dorado receives full damage bonus, Avengers can flex their offensive power with full class advantage, but any powerful sustained ST nuker such as Robin or Kintoki (Rider) will have the damage required.
Defense Up buffs work well to counter the large amount of incoming damage, but Megalos's own Attack buffs can counter defense stacking strategies. Hard survival options are most preferable, but healing is least effective. Megalos's damage output is simply too high. Taunts are great for protecting key DPS Servants and can even attempt to redirect Megalos's Taunt debuff (which can be cleansed too).

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.

Video #1 - Provides a thorough look into the fight over the course of 15 turns.
Video #2 - Good video to watch to understand how Megalos' Taunt works with respect to Megalos' multi-hit attack. Note that it shows information on more than just 11-5, so be careful of spoilers.
Video #3 - An alternative look at the 11-5 battle.