More and more AP Sejuani Toplane OTPs are appearing in master-grandmaster korea: https://www.onetricks.gg/de/champions/ranking/Sejuani

Especially after Recent Buffs in 14.9 Sejuani Top is back, STRONGER THAN EVER.
Sejuni has not only GREAT LANE, being able to stomp many opponents in a VIDE variety of matchups, but also really strong early/mid skirmishes and lategame teamfight usefullness
Sejuani is also a GREAT blindpick champ with only a few matchup that can be handled

I managed to reach Diamond EUW without ANY problem mostly thanks to this pick:

In Early Levels 1-5 you want to let the opponent push you in while trimming the wave slightly, setting up a freeze if possible and poking your opponent with W, Comet + Scorch. (Be careful not to push the wave too much)

Once you get Ionian Boots you can W -> W without the Stacks falling of.
With this build Q is mostly used to disengage while u try to space your opponent and poke with W.

You Spike a lot at Level 6 so wait it out in matchups where you seem to struggle pre 6
Seju is a Mid Game Powerhouse and this Build enhances he midgame even more. Levels 7-12 (when your W gets maxed) You will outperform almost any Toplaner in Skirmishes

I know this is in Korean but its still nice to see some of his Gameplay:

Korean video on Early Lane

Early Skillorder most Games: W -> Q -> W -> E -> MAX W -> MAX Q

These OTPs have huge variety in their builds and even the core Items, which I had a hard time understanding before. After large amount of testing on a fresh account (100+ Games in Silver-Diamond EUW) and watching other Sejuani OTPs i have found the 3+ main build variations:

1.Liandries first:
Probably most reliable all around as you get really tanky later. Good into Tanks/Juggernauts. More expensive than Rylais but very good One-Item Spike.

2. ROA -> Liandries: Highest Dmg, best Skirmishes, worst 5v5 due to lack of tankiness, especially bad vs lots of max HP dmg, good vs Teams with few tank and many squishies (i.e. good vs Jax, Irelia to completely destroy them in lane and snowball) Mejais, Rabadons are good if you get fed, turning it into more of an assasin build, otherwise go tank with i.e. Kaenic/Frozen Heart

3. Rylais Rush: Great into ranged matchups or where slow is really good (i.e. Jayce). Cheapest 1st Item Spike, lowest Dmg https://youtu.be/sV_LRKjlt84?si=bkRZa4JHhR9uAnG9

OVERALL: In most Games going Liandries OR Rylais -> Fulltank is better than building BOTH Liandries + Rylais. Since Sej Buffs her HP Scalings are great so IBG/Wamogs/Kaeanic also give good Damage AND nessecary Tankyness

IF the enemy team has low dmg/you have another good Tank already AND you are not getting hit much in Teamfights OR youre snowballing really hard OR Rylais Slow would be really usefull that game (really Crippling some champs/Teamcomps) THEN and ONLY THEN should you go Both Liandries + Rylais

Credit to Chitrr (Master Seju Top): https://youtu.be/gW-i4XGiaRo?si=VlKjf784slZk2yoZ

Both Top and Mid are fine but i prefer it against melee champs who can be spaced easily.
Mabye even more important than the 1v1 is your teamcomp and jungler.
Seju excells with a catch-type teamcomp utilizing your Longrange Q-R Engage and Burst that comes with it.

So dont be afraid to walk around with your team alot, ping ALOT and be the one looking for picks and catches

Seju doesnt have THE greatest 1v1 sidelane strength so champs like Garen/Jax/Fiora will onescale you at some point in the 1v1 but you will be 10x the champ they are in teamfights and early 2v2/3v3 skirmishes

Runes: Best Overall Setups is Comet, depending on Matchup, Grasp is better if you need to be able to contest in AA range: (Yasuo, Yone, Renekton)
Comet is better in most cases because it gives great dmg all throughout lane and therefore more oportunities to solokill. Also in Lategame the extra Damage helps ALOT when trying to oneshot an ADC with Q-R snipes

Comet, Manaflow, Transendence, Scorch
OR
Grasp, Demolish, Second Wind/Flex, Overgrowth

Secondary should always be Cosmic Insight + Cookies/Cashback
(Cookies for better lane, Cashback to reach 6 Items faster)
Shards: Cdr + AP + Flat HP

Another korean OTP takes Aery using the Rylais build, this could be a whole other debate on its own but in most cases it comes out to be similar/slightly less damage as Comet. I like aery vs Teemo/Singed as they can walk out of coment too sometimes

AS OF 14.12 Firststrike is no longer as Great. Before the nerf it was good in Matchups like Shen where you can freefarm it on the opponent and you probably wont kill anyways. You would go the Full DMG & Burst build with ROA -> Liandries here