Boss: Demon God Phenex (Caster) Ch16-2 (Agartha)

Enemy Layout

Battle 1/1

709,894

Demon God Phenex

Drops	Spirit Root x1 Pt 1 Lamp of Evil-Sealing x1 Pt 1 Warhorse's Young Horn x1 Pt 1 Serpent Jewel x1 Pt 1 Phoenix Feather x1 Pt 1 Eternal Gear x1 Pt 1 Ghost Lantern x1 Pt 1 Evil Bone x1 Pt 1 Caster Monument x1 Pt 1 Void's Dust x1 (300%) Pt 2 Mysterious Divine Wine x1 Pt 5
Rewards	Holy Grail x1 Unlocks Rider of Resistance in Story Summoning. Profile Update for Assassin of the Nightless City.

Active Buff		Increase Damage Resist against enemies with [King] trait (Cannot be removed).
Skill 1		Inflict Charm on a single enemy (1 turn). Inflicts a delayed debuff that decreases target's HP by 2,000 the next turn.
Skill 2		Decrease Death Resist for a single enemy (? Turns).
Skill 3		Chance to inflict Death on a single enemy.
NP Effect		Deal damage to all enemies and inflict Burn (3 turns, 1000 dmg/turn).

Phenex’s normal attacks will hit everyone, thus it can stack up quickly without damage mitigation such as Defense buffs or class advantage. This also makes bringing Taunters incredibly ineffective.
Party-wide Evasion or Invincibility work wellfor avoiding Phenex’s Noble Phantasm, or to buy a turn when team’s party HP is low. Phenex's NP isn't a total party-kill, but without protection it will deplete nearly a full HP bar on neutral SSR Servants.
The Burn damage from Phenex’s Noble Phantasm can stack and build up over the long term. Masters that prefer more stalling-centric tactics should take this into account for a prolonged battle. High debuff resistance can also stop the burn from being applied or Masters can absorb or cleanse the debuff.
Be mindful of one of Phenex’s skills that can cause a Servant to lose 2000 HP on the next turn regardless of protections, especially on Servants with low HP revival on their Guts.

Generally, a team of 1-2 Riders with 2-1 Supports should be the go-to team for this quest.
Alter Egos, and to a lesser extent, Berserkers, can also work as DPS options, but they lack the class advantage resistance of Riders so Masters should take extra care when fielding them.
As Phenex has passive resistance to Servants with the King trait, Masters could avoid fielding these Servants as main damage dealers. The damage reduction is very noticeable whenever an NP from a King is used, but some of them still function quite well.
Phenex's overall damage can be successfully stalled through with Arts compositions. However, the Instant Death aspect of his kit may decide to randomly take out Servants.