Boss: Rider of Resistance & Megalos Ch14-2 (Agartha)

Enemy Layout

Battle 1/1	203,927 Resistance Rider	308,340 Megalos
	148,762 Resistance Rider	250,740 Megalos
		205,060 Megalos

Chapter 14: King of Slaves AP - 25

Drops	Rider Monument x1 Pt 2 Berserker Monument x1 Pt 2
Rewards	Saint Quartz x1

Boss Statistics

Megalos
	HP	308,340
	LVL	90
	HP	250,740
	LVL	90
	HP	205,060
	LVL	90
	Class	Berserker
	NP Bar
	NP Type	AoE
	Actions	Up to 3 per turn (2 while Rider is alive)
	Traits	Servant, Male, Humanoid, Divine, Greek Mythology Males, Super Giant, Weak to Enuma Elish

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). Note:He will use this skill frequently whenever he reaches a damaged state.
Break		Increase own Critical Strength (3 turns). Decrease own Quick Resist (3 turns).
Break		Increase own Critical Rate (3 turns). Decrease own Arts Resist (3 turns).
NP Effect		Deals heavy damage to all enemies.

Resistance Rider
	HP	203,927
	LVL	80
	HP	148,762
	LVL	80
	Class	Rider
	NP Bar
	NP Type	AoE
	Actions	Up to 3 per turn (1 while Megalos is alive)
	Traits	Male, Humanoid, Servant, Riding, Evil, Weak to Enuma Elish

Skill 1		Voyager of the Storm B: Increase all allies' NP Damage and ATK (1 turn).
Skill 2		Unyielding Will C: Apply Guts to self (1 time, 3 turns). Increase own NP Gauge by 1.
Skill 3		Conquistador EX: Increase own Buster Card effectiveness and Critical Rate (3 turns).
Break		Apply Ignore Invincible to a random ally (3 turns).
NP Effect		Santa Maria - Drop Anchor: Deals heavy damage to all enemies and inflicts Crit Star Drop Rate down (3 turns).

Overview

This is the trickiest fight in Agartha, yet also one with multiple solutions depending on what a Master's roster excels at.
There are 5 key aspects:

Rider of Resistance will buff Megalos with Invincibility Pierce if his HP bar is broken, nullifying any hard survival skills for its 3-turn duration.
Rider of Resistance will buff himself with Invincibility Pierce for 3 turns if Megalos already perished.
Megalos absolutes loves to use his Evasion if someone brings him to a low HP range on any of his Break Bars, slowing the fight down immensely.
Rider of Resistance can charge his NP bar, causing their NP Bars to no longer be synced.
With additional Critical buffs on Megalos, a Rider boss, and an AoE Rider NP (that will kill), Caster supports are decidedly fragile.

The key element in this fight is triggering the Invincibility Pierce in a safe manner, while either rushing the fight with sheer damage, or sustaining a team throughout the Megalos onslaught long enough. Based on the above there are 3 main ways to victory:

Rush Megalos, pouring on the high damage to take him out before Rider of Resistance can NP more than once. Craft Essences or skills that pierce Evasion are ideal! Alternatively, he will not spam his Evasion if Masters "simply" take out his entire HP bar in one go, thus never letting him go low HP.
Rush Rider of Resistance, activating his Invincibility Pierce in the first 2-3 turns upon Break. Masters can stall Megalos during the duration and let it expire before he charges his NP, ideally having taken out Rider of Resistance before he can NP.
Rush neither, but time Rider of Resistance's first Break Bar in a way that no NP will be launched while the Invincibility Pierce buff is active.

Team Recommendations

While relying on the same composition as was used during the previous Megalos encounter is still very viable, especially in terms of defenses, Mash really stands out with her class neutrality. Her buffs can enable Caster supports to survive long enough to bring the fight to a close.
Since Megalos can still do an immense amount of AoE damage and can a apply a Taunt of his own, Taunt Servants can be effective Lightning Rods during Invincibility Pierce, but it is not a safe, clear cut answer.
All Anti-Trait effects for Megalos are still in effect, with Quick Servants such as Scathach especially effective given Megalos gains a Quick demerit after first Break (and an Arts one on second).
Assassins such as Cleopatra or First Hassan are great for quickly disposing of Rider of Resistance especially with powerful Buster Critical Support.
Don't forget about any stored Craft Essences with Sure Hit or Invincibility Pierce. They are powerful options to ignore Megalos's exceedingly annoying Evasion use.
Stuns, Charms and NP Drains are great for stalling Rider of Resistance whenever he does decide to NP Charge. With good timing, Masters can still tank both NPs with a single Illusion.

Video Links

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: A cheap Euryale composition that frags Megalos quickly.
Video 2: A Cleopatra composition that focuses on Rider of Resistance.