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

Especially after the Buffs in 14.9 Sejuani Top is back, STRONGER THAN EVER.
Sejuni is a blindpickable champion, meaning she has no UNPLAYABLE Matchups. You can go both Tank and AP Offtank to adapt to your Matchup/Teamcomp needs.
In Terms of Playstyle Sejuani is like an improved and more flexible Comet Malphite.
The way you Poke with your W and oneshot them once they are low. Except unlike Malphite you have more Mobility, More stuns, a selfpeel knockback on Q and more 100 to 0 burst potential with your Ult. (More in the Early Lane section)
Many Matchups such as Jax, Jayce, Irelia, Garen are all Matchups, that a seasoned Sejuani Player will be able to STOMP EASILY (More in the Matchup section)

I managed to reach Diamond EUW with a 76% Winrate without ANY problem mostly thanks to this pick:
https://imgur.com/b82s6Za
https://imgur.com/ZUEGMuj

In Early Levels 1-5 Sejuani isnt the strongest champion but with Comet + Scorch and starting with Dring + 2 Pots you can savely poke and get CS whereever you can.
Every Point Sejuani puts in her W ability increases her dueling strenght massively!!

You want to RUSH Ionian Boots, so that you can W -> W without your E Stacks falling off.
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.
Your full Combo: W -> W -> E -> Auto -> Stepback & Ult -> Auto -> Ignite will do around 70-90% of most Champions at lv6-7. Save your Q if possible so you can follow them after your combo if they flash away with Q + another W.
Seju is a Mid Game Powerhouse and this Build enhances he midgame even more. ESPECIALLY once you reach LV9 Sejuani will outperform almost any Toplaner in 1v1/Skirmishes. At this point Roaming midlane/Fighting in the River with your jungler will almost always be a garuanteed WIN!!!

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
Alternatively if you have a matchup where the enemy wont punish you for getting your E fully stacked you should go W -> Q -> E -> MAX W

As of Patch 14.13-14.15 Sejuani wants to Abuse Warmogs like many other champs do.
He W scales massively with HP but getting some early AP help with Killpressure in Lane.

You want to start Dring + Pots.
First Back get Darkseal/Ionian Boots
First Item will be Rylais EVERY GAME (Except for Matchups/Games where you should go Tank)
Why Rylais:

1. Rylais is incredably Gold Efficient for Sejuani. Your Champion wants only 2 Things AP & HP (thats what your W dmg scales with) and Rylais gives you alot of both (at 104% gold efficiency)
2. The slow is great for guaranteeing your W2 Hit which is the majority of your W Damage
3. The wide 180° AOE slow especially from your W1 gives you IMENSE Teamfight Utility/Spacing Ability in lane & Catch Potential (Especially vs Ranged)
additionally its great for running people down the long toplane
4. Its cheap and segways perfectly into Wamogs second item

Second Item is Warmogs then you can go Zekes or other Tank/Support Tank items

Tank Alternative is Grasp with Tear/DShield start, early Fimbulwinter/Sunfire into Tank Items

For future refferences/experimentation:
IN the Past you used to go ROA -> Linadries (the full dmg build)
then some time later you used to go Liandries -> Rylais

Both these builds are still decent and very fun to play but the Insane amount of Utility you gain from Rylais and Warmogs healing just trump those build by miles!!

Additionally you used to be able to go Firststrike against some champs like Mundo Shen Ornn and just farm free gold of them. Sadly Firststrike got nerfed into an unusable state so its not good for now!!

Both Top and Mid are fine but i prefer it against melee champs who can be spaced easily. In Midlane you want to look for Squishy champs, so stuff like Ryze Cassio are a nogo

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. (Especially good vs many Squishies)

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 once the Game goes to 3+ Items. This means that Champs like Garen/Jax/Fiora will onescale you at some point in the isolated 1v1 but you will be 10x the champ they are in teamfights and 2v2/3v3 skirmishes.

Best Overall Setups is Comet, depending on Matchup Grasp & going Fulltank can be better (look at Matchup Table)
Comet is better in almost ALL 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
Secondary: Cosmic Insight + Cookies
Shards: Cdr + AP + Flat HP
This NULLIFIES Sejuanis only potential weakness which is early levels 1-5

Very RARELY i recommend AERY, as the dmg is very similar but some champ can dodge the coment too consistently, so you just go aery instead

Outdated Text below:
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

If you look at some Statistic Sites like Lolalytics you will quickly realise, that Tank Sejuani Grasp + Tank has a GOOD amount of higher Winrate than AP Sej.

1. Playing Tank Seju is WAY easier. Playing Sejuani with DShield + Second Wind you have almost 0 Risk in that Game. LV 1 you can chillmax and trade with Q + Graspauto and win many 1v1s. Once you reach 3-4 Items you are just a unkillable Meatball of tankiness

2. Learning Curve and POTENTIAL of AP Sej
If you look at Sites like onetrick.gg and search for Sejuani you will see AP Sejuani players raining supreme in the Top Positions. This is because OTPs have mastered Sejuani and know the Potential of this champion when going a more Damage oriented Style. Instead of being a Lane nutralizer AP Sejuani is a SOLOKILLING DEMON.
VERY frequently you can see seasoned AP Sejuani Players go 15/2 in High Elo Games.
Knowing your Damage Potential once you reach lv6-9 can be the difference between going 1/7 or going 15/2.

3. Videos like Loldobbys video or Random players getting inspired by this OTP deflate the ACTUAL winrate of AP Sejuani. They see the build, see the success that some are having with it and go right into ranked and INT their ***es off.
Dont be that guy play 10-20 practice Games and you wil see IMENSE progress in you AP Sej Games

4. Best of both Worlds
IF you want to become a REALLY GREAT Sejuani Top player you should try to learn BOTH AP and Tank Sej.
Experiment with both Tank and AP in many matchups and see which feels better, AND mabye try to adapt to your Teams needs!!! If you have 0 Tanks 0 Engage 0 Frontline just go Tank and play for peeling your Carries!!

5. TRY To experiment and see what works best for you but give it some time.
I personally started playing Sejuani when she was a meta Tank Toplaner with old Frostfire Gauntlet. AT the time she was really OP and could statcheck almost any Toplaner in the 1v1.
Then she fell out of grace largely because of proplay.
Once i picked up AP Sejuani at first I was REALLY struggling hard even against very bad players, because i wasnt used to not being allowed to play in melee range and trade auto with my opponents.
50 Games later AP Sejuani had become BY FAR my best champion and I still feel like im improving to this very day