I'm in bronze and I made this for fun. I don't even have m7 on Seraphine. I just run this because I get overpowered in teamfights.

Pros

Team lives forever
Can take objectives with 2 people due to crazy shielding
CC chains
Big *** ult
Really good out of laning phase
Squishies are no longer bursted instantly
Tanks get a lot of healing due to %max health healing

Cons

No damage
Entirely reliant on team to do damage
If you die, your team no longer immortal
Range is limited and spells can be dodged easily
Limited to cycle on W

Summon Aery
Very good for shielding, must take
Manaflow Band
Mana for sustain
Transcendence
Ability haste for W spam
Scorch
Good early game poke in laning phase
Bone Plating
Good peel in laning phase
Revitalize
Most important part of the runes, need that extra 5% heal/shield power

Passive: Seraphine gets empowered autoattack and more range depending on ap and amount of allies nearby. Seraphine gets an automatic recast or "echo" when she has cycled 3 spells. Her abilities have special things that happen when someone has already been hit with a status effect from her allies or her own prior spell.
Q deals more damage to lower health enemies, is your main source of damage.
W heals when Seraphine is already shielded.
E slows, roots if already slowed, and stuns if already immobilized.
R charms, and extends if any champions, enemy or ally, are hit.

Play aggressive. Your base stats and abilities are super useful. Seraphine Q is insane in earlier levels and along with Echo's recast, you do so much damage. You will be playing with less damage and more shielding later so get your adc ahead while you can. Taking Scorch for the extra damage is important for this, and taking Bone Plating will help you peel for adc when playing aggressive. You can even use your Exhaust + E combo to run people down.

Your teamfights go crazy. You keep cycling W with the Echo recast and your team gets so much shielding. This is also why you level up Q after W, because Q gets reduced cooldown per level, and that will help you cycle faster. Also, remember that your ult EXTENDS after hitting people, even if a person is further away, if you hit them in a line, you can hit some crazy ults. Seraphine ult was literally made for teamfights. Make sure to follow up ult with E for combo.

Me personally this season, due to the removal of chemtech putrifier, I feel that buying antiheal has become a huge pain in the *** for support enchantresses. Morellonomicon is the only replacement for antiheal on mages, but it doesn't give heal/shield power, which means less shielding for the team. If I can avoid building antiheal, I would, but if there are any lifesteal based champions you kind of have to. Also, if you feel like you are not quite shielding enough, selling boots for another item is an entirely doable thing. Seraphine W gives her move speed anyway, and support items will also have ability haste so you won't be missing out on much.

Thanks for reading and have fun!