Strengths:

[*] Versatile; Has good mix of utility and damage and fits well with any lane partner/team comp. Can lean into being more of an AP damage dealer or an enchanter depending on what the team needs.
[*] AOE Shielding / Healing
[*] All ability damage is AOE
[*] Great teamfighter, late game teamfight monster
[*] Lots of crowd control
[*] Very good range
[*] Strong Ultimate
[*] Scales well, Useful from behind
[*] Easy to play, but still skill expressive with 3 skillshots
[*] Gorgeous with stunning skins

Weaknesses:

[*] Extremely squishy, limited mobility
[*] Relatively weak early game
[*] Spells are slow and telegraphed; Weak against very mobile enemies
[*] Long cooldowns, especially R and W
[*] Mana hungry
[*] Can be difficult to get full use out of R

Seraphine is a well rounded champion who can adapt to always be useful to her team. Her W and notes passive become much stronger for each nearby ally, therfore she is best suited to the duo lane. Her biggest strength lies in late game team fights where both teams are stacked, because she can get crazy value out of her W, R and even CC from E while providing respectable AOE damage. She can really be behind the whole game and win through her utility in one crucial late game teamfight. However, Seraphine is very easy for enemies to shut down if they focus her, especially in the early game. It can be very difficult to play Sera in lane if the enemy jungle is hyperfocused on you. Enemies that are very mobile can also dodge her spells with ease and catch up with her to delete her.

Catchers are a support champion archetype in League that focus on providing strong crowd control. Examples of other catchers are Zyra, Blitzcrank and Morgana. Each has at least one signature CC ability that defines their playstyle and allows allies to pick off vulnerable enemies. Similar to Zyra and Morgana, Seraphine has a long range CC ability and an AOE CC Ultimate. I believe that with the recent changes to Seraphine's E, it is worth maxing first for a catcher playstyle. Points in E now buff the slow/root/stun duration, lasting 1.5 sec at max rank. Combined with Rylais, this gives a long lasting hard root that goes through enemies, similar to Zyra's E. The high base damage on the ability also makes it a strong poking tool in lane. This allows Seraphine to function as a mid-high damage catcher with high utility. Focusing on crowd control provides high utility (as opposed to full dmg) while also playing to one of Seraphine's real strengths (as opposed to full enchanter).

Early game / Lane: Focus on poking while also healing and disengaging for your adc when they require it. Sera isn't the strongest early game but she isnt terrible either.

Mid game: Once you complete Rylais your power will spike hard. Stay grouped and focus on providing good cc for allies.

Late Game: Stay with team always. One good late game teamfight with Sera could = win. If you have someone popping off on your team you can follow and protect them.