Sejuani is a ad teams nightmare. Not only does her health and armor scale really well, but her damage and crowd control is crazy. Her (Q) Arctic Assault scales on enemy max health and her (W) Flail of the Northern Winds scales on her max health. Her (Q) Arctic Assault is also one of the best defensive skills in the game as it can go over any wall on the map or can be used offensively for a long distance gap closer. Her skills allow for great team fights in which she can have multiple targets stunned and slowed at the same time. Sejuani in my opinion is a very under rated champion and hopefully will be picked up more.

All Items purchased should have either a mix of health + AP or Armor + AP unless you get a deathcap. Rush Zhonya's Hourglass if you laning against Zed, or if you get ahead and want to scale into late game rush a Rod of ages. Iceborn Gauntlets are also a great item as they provide even more crowd control, ability power, much needed mana, and armor. Once you get a Rabadon's Deathcap you should be able to blow up the adc or support on the other team or any squishy targets for that matter. Ninja Tabi boots will provide more needed armor for tanking. With these items Sejuani becomes the rare AP tank.

FROST ARMOR:
Her passive is probably one of the best in the game in my opinion. It provides all abilities and attacks a 20% movement speed slow and increase her armor every 5 levels after level 7. This makes her a great front line and a amazing peeler for carries.

ARCTIC ASSAULT (Q)

Arctic Assault is a great ability for both engaging and disengaging. It has a very long range and the damage it does scales very well with both ability power and max enemy health combined. It also has a knock up for added CC.

FLAIL OF THE NORTHERN WINDS (W)

When it comes to ability combos this skill you will probably use last as you need to be able to stick close to the target for max damage. With items that give Sejuani lots of health, this skill get stronger. Max it last in as you will need max slow and cc for Flail of the Northern Winds max damage.

PERMAFROST (E)

Permafrost is a great slow. at level 1 is slows for 50% and at max level 70%. Try your best to hit multiple targets with your other abilities before using permafrost as it will damage and slow every target with Sejuani's passive applied.

GLACIAL PRISON (R)

Sejuani's ultimate is just another great tool she has for engaging or disengaging. It has very long projectile range and if it his a target its stuns all enemies in a small area. If it misses it slows for 90% in a large area. With this ability along with her (Q), Sejuani can easily catch some of the slipperiest of champions.

These runes are based on personal preference. I personally like scaling glyphs of cool down as it provides 15% cd at level 18. you can change this if you want but at level 18 all you 3 core abilities are on a 5 second cool down and your ultimate is on a 60 second cool down. The rest of the runes seem pretty standard for early game dominance.

Farming against a ranged champion can be very hard but only if you are greedy and want to take damage for getting a few creep kills. Sejuani's (Q)+(W)+(E) combo can delete an entire wave fairly easy. Wait for your enemy to push your wave and just combo the creep wave. If you can get the enemy champ in the combo as well you might be able to get a kill especially if you can catch him near your tower.