Phainon is a Physical Element Character that follows the Path of Destruction. He is an extremely powerful DPS when he is at his Enhanced State, he can tank, aggro, counter, and unleash devastating amount of damage. His kit revolves around stacking his Coreflame to activate his Enhanced State via ULT: this requires him to funnel all buffs, debuffs, and enemy hits unto himself. His playstyle makes him easily good at Hypercarry Teams involving multiple harmony characters.
Best Cavern Relics
(2) Increases CRIT DMG by 16%.
(4) When the wearer becomes the target of another ally target's ability, gains 1 stack of "Help," stacking up to 2 time(s). If there are 2 stack(s) of "Help" when the wearer uses their Ultimate, consumes all "Help" to increase the wearer's ATK by 48% for 1 turn(s).
The best relic set for Phainon, while having 2 harmony characters in the party allows him to quickly max the "Help" stack. This, when ULT is unleashed, will enhance his ATK by 48% until the end of his Khaslana mode (Enhanced State).
(2) Increases CRIT Rate by 8%.
(4) Increases DMG dealt by Skill and Ultimate by 20%. After using Ultimate, additionally increases the DMG dealt by the next Skill by 25%.
This set is comparatively good for Phainon, it permanently increase Phainon's Skill and ULT DMG output by 20 % and additionally buffs his next Skill DMG by 25% after his ULT.
(2) Increases Physical DMG by 10%.
(4) After the wearer attacks or is hit, their ATK increases by 5% for the rest of the battle. This effect can stack up to 5 time(s).
This set can be considered as it improves Phainon's Physical DMG by 10%. It's ATK% stacking however requires him to get hit by enemies up to 5 times, making it slower to reach it and requires multiple enemies for faster buffing. Regardless, the stack will not disappear unless Phainon gets fatal damage or when the battle has ended.
Best Planar Ornaments
(2) When there are currently more or less than 4 ally targets in battle, each additional/missing ally target increases the wearer and their memosprite's DMG by 9%/12%, up to a maximum of 4/3 stacks.
The Arcadia of Woven Dreams is tailored made for Phainon as this set's buff can be maxed out upon his ULT activation.
(2) Increases the wearer's CRIT Rate by 8%. When the wearer's current CRIT Rate reaches 70% or higher, the wearer's Basic ATK and Skill DMG increase by 20%.
This set essentially increases Phainon's Crit Rate but requires him 70% Crit Rate to trigger the Basic and Skill DMG. Using a Crit Rate body and decent crit rate substats makes this easy to achieve.
(2) Increases the wearer's ATK by 12%. When the wearer's SPD reaches 120 or higher, the wearer's ATK increases by an extra 12%.
This is an easy set with easier buff mechanics and also fits with Phainon since it increases his ATK as long as he has 120 SPD and above.
Best Stats

Body
CRIT Rate

Boots
SPD

Sphere
Physical DMG Boost

Rope
ATK%
Substats
CRIT Rate / CRIT DMG / SPD / ATK%
info
Phainon usually requires 2500 ATK, 135 SPD, and 60/170 Crit Ratio. He receives a relative amount of Crit DMG on his Traces and can rely with support characters to enhance his Crit DMG especially Sunday, Robin, Remembrance Trailblazer, Sparkle, etc.
Best Light Cones
SourceLight Cone Event Warp
Dispossession
The wearer's base SPD increases by 12~20. When the wearer deals DMG, ignores 18~36% of the target's DEF. After the wearer uses Ultimate, obtains "Blazing Sun," which is removed at the start of their turn. While "Blazing Sun" is in possession, increases the wearer's DMG dealt by 60~132%.This is Phainon's Signature 5* Light Cone, it initially allows him to ignore enemy's DEF. Upon ULT activation, Phainon will deal 60% more DMG, this makes his Khaslana Mode more devastating than ever.
Moth To Flames
When the wearer attacks, increases their ATK by 8~16% in this battle. This effect can stack up to 4 time(s). After the wearer breaks an enemy's Weakness, increases DMG dealt by 12~24%, lasting for 2 turn(s).This LC is an F2P choice and easily redeemable via Herta Store. It enhances ATK and additionally provides DMG when Phainon triggers Weakness Break.
Massacre
Increases the wearer's CRIT Rate by 12~20%. The Skill DMG and Ultimate DMG dealt by the wearer increases by 24~40%.This can be obtained by purchasing the Nameless Honor at LVL 30. It naturally provides Crit Rate and enhances SKILL & ULT DMG.
SourceStellar Warp, Light Cone Event Warp, Starlight Exchange, Stellar Convergence
Kinship
Increases the wearer's ATK by 24~40%. When the wearer defeats an enemy or is hit, immediately restores HP equal to 8~12% of the wearer's ATK. At the same time, the wearer's DMG is increased by 24~40% until the end of their next turn. This effect cannot stack and can only trigger 1 time per turn.This is comparatively good for Phainon, it initially improves his ATK and restores HP and buff his DMG whenever he defeats an enemy until next turn. Defeating enemies in consecutive actions does not stack these effects.
SourceStellar Warp, Departure Warp, Character Event Warp, Light Cone Event Warp
Rye Under the Sun
Increases the wearer's ATK by 16~32%. When the wearer defeats an enemy, the wearer's CRIT Rate increases by 12~24% for 3 turn(s).This LC buff Phainon's ATK and improves his Crit Rate whenever he defeats an enemy for 3 turns. The latter can be easily triggered if the party fights multiple enemies.