I actually fell in love when I heard about her spells, I already felt how she must be played and spammed her in midlane. So the reasons for me Seraphine is good to play are:
-Easy to play: No more mechanics to learn allows us to spend more time on maccro game

-Basically the strongest champion for teamfight: No one can challenge her strength: Game changing ult, a redemption+Karma's shield in ult+ Sivir's ult every 8 seconds in late game, you can't contest this.

-Her clear wave is completely stupid: If you didn't take the dematerializer, if you spent around 1700 gold in ap (lost chapter or such), in lv 8 you can finish the caster minions with a double Q and E or double E and Q, which make it hard for the enemy to push a tower if they don't have a nashor in a 5v5.

-She makes music, and I love music ;(


Her weaknesses: She uses a lot of mana in early and her spells are very easy, maybe they are part of the easiest spells to dodge, so in early you will rarely touch your oponent at long range. She is fragile the whole game but you suffer the most about it in early. Survive the early and you can have fun :)

What I propose here is not how you HAVE to play her, I think that since the runes were reworked a few years now, you can create your own style with any champions. I just share how I like to play Seraphine and explain why I think it is efficient on her. You don't have to follow what I do, you can just read it, try it if you wish and then make your own idea.

I am just a gold player but I try to learn from others (from Iron players to challenger), or at least listening to them. Anyone deserves to at least be considered, so don't hesitate to talk to me about my choices if you ever come here and I will do my best to make you learn few things, in exchange, I expect you to do the same with me :D

Also it is the first time I do a publication here, I might do one for my beloved Poppy as well but I hope to have some responses!

Seraphine can look like Lux or Sona, but her gameplay is kinda different to them, especially Sona:

Early game:as a long ranged mage, her early will be passive: Farm, Poke, Ward when you can to avoid getting ganked and wait for your jungler to be aggressive. The only moment she can be abused is the early game, you CAN'T loose your early. Seraphine is not made to win the early either, but we can explain her lane easily with one phrase: "your goal is not to win the lane, but to not loose it.". It doesn't matter if you don't kill your oponent, as long as they aren't fed it is not a problem, you just want to reach the midgame. Don't die for kills, if your enemy survive with 1 hp, don't risk your life to kill them, 1 kill for your oponent can doom your early/midgame. The only moment you will move from your lane is to help your jungler and hope to get a kill. You are too defenseless to go further than the scuttle.

Midgame: At the moment you reached your lv 6 you have 3 possibilities: You won the early, so you abuse of your strength and try to roam with your Jng or to other lanes but ONLY if you have enough visions, you are in a neutral game and your oponent doesn't roam, so you play the post lv6 like the early, and the enemy roams but you cant contest, so you push and try to get plates, just make sure to signal them.
If you are confident in your ability to counter gank them or to punish them you can follow them, but if not don't, try to reach your corebuild. After this, the machine will already be ready, and when the laning phase will be over, you will turn from a weak champion easy to abuse to an untargettable monster in teamfight.

When the teamfights will start, your job will be simple: poke at long range and try to find openings, heal your allies and cast the best ultimate possible. Define your target and as soon as these targets can be caught and your team can follow you, don't hesitate and use your ult. The amount of person you hit with your ult doesn't matter, what matters is that you hit the right ones. Except this, you have nothign else to worry about for the lategame.

DO NOT SPLITPUSH WITH SERAPHINE, she is easy to 1v1 since she only has skillshot, you can stop a wave from destoying a tower (she has one of the best clearwave of the game), but do not ever splitpush.

As I saw with Cupid, the R+Flash works and can give a kill easily, if you are confident in your skill you can try it, it will just allow you to send it from further and hit an enemy that might have been too far at first.

Even if Serapine doesn't need a red buff, don't be sad at all when you have it: it synergise very well with your E: the red buff will slow enemies which make your E root/stun your enemy. Moreover you can easily be more aggressive. So if you ever get a red buff, try to auto attack as much as possible your enemy to make sure you can cc them.

The Everfrost (and maybe other activable items but I only found it with Everfrost) gives Seraphine 1 note of her passive, it can look futile but her notes were buffed after the level 13 and that can make a difference since it deals way more damage to minions, it gives a bigger range and such. It looks minor but I am sure you will find opportunities to use it :3

Seraphine's heal with W still works when you are in stase (Zonyah, Bard ults). So you can double W or W with a shield to get the heal anyway without getting killed and, consequently, being not able to heal your allies.

Every single drake soul are strong with Seraphine: the infernal drake increases her AP so she heals/deals more damage, the ocean drake soul gives her more sustain and heals, the cloud drake allows her to spamm her ult every 40 seconds, but my favourite one is the mountain drake: the shield granted by the soul is a pure gift for the game: not having to double W or to use your bareer and W to grants a strong heal every 8 seconds gives us a bunch of new possibilities since you can now focus on using double E or double Q.