Review

While Poseidon can do good damage, her true potential only unlocks at SSS with high transendence functor. If you already have a built Izanami or Skadi, you can skip Poseidon; otherwise, Poseidon can be a solid Ice DPS option.
1. Functor: The signature functor is basically a must-have. It provides constant Energy recovery for Poseidon to become a true skill machine gun. Increasing the transcendence rank also has very high value.
2. Sigil: Slot 135 choose [Vanargand's Seal]. Slot 246 choose [Song of the Victor]. [Iceberg] or [The Story Ends Here] can be very niche options for support playstyle, but it is generally not recommended.
3. Enchant: Prioritize 888, followed by Tactics Enhance and Ice Blessing. Calculated Reserve is recommended to reduce her Energy usage. There is no need for dodge related enchants after AKS.
4. Warp: Basically all of Poseidon damage is from her S1, so slot 12 choose Flashback Core I + Power UP: Ranged / Evolution Particle I. Slot 34 choose EM Flux + Telepathize Forcefield I. Slot 56 choose Current Decay + Aether Shatter / Absolute Zero (S Hera).
5. Aether Code: Blue 3 is the best choice for DPS Poseidon. Red 3 is an Ice support choice; however there are better support options, and Poseidon's AI is not good. Yellow 3 has high burst potential; however, it has dodge issues and shorter Edgy state, so it is only suitable for very niche speedrun cases.
6. Skill Stat Boost: Only her S1 is recommended. S2 and S3 are optional because they have extremely poor returns.
7. Rotation: Cast S1 twice to gain a Snowy mark, cast S3 to gain another Snowy mark, then cast S3 again to enter Edgy state. While in Edgy state, cast S1 and dodge right after repeatedly. You should cast S2 on Elites and Bosses if they are not in CC immune state for Current Decay and [Vanargand's Seal]'s activation.
8. M.E.O.W: Bring Me-Yow so that Hera ZT can grants even more Energy.
9. SS vs SSS: At SSS, her equipped warps gain +1 level, making Flashback Core I become level 7, which reduce her S1 CD even further. This helps Poseidon spam S1 in Edgy state without downtime.