When ganking or in smaller fights, try not to initiate with Arctic Assault if possible. Save it for chasing down an enemy who tries to get away with flash or other escapes.
Northern Winds grants you decent damage on a target you are focusing and have applied Frost to.
When jungling, learn Northern Winds first as you can still get the bonus damage off on the large monsters since your attacks apply Frost.
Use Permafrost after you land a basic attack or charge in with Arctic Assault. The slow increase makes it easier for Sejuani and her allies to pick off the enemy.
Your ultimate Glacial Prison is your only hard crowd control effect, making its proper usage vital.
As Glacial Prison applies Frost to enemies hit in a large area, you can intiate fights by combining it with Arctic Assault and Permafrost. After your stun enemies will still be nearly immobile, giving your team a 4-second advantage in which to crush key targets.
When farming minions, first charge through as many of them as you can with Arctic Assault to apply Frost to as many as possible to get bonus damage on all of them with Permafrost and Northern Winds.
Build usage
Building both Frozen Mallet and Rylai's Crystal Scepter at the same time is not recommended. Frozen Mallet is typically the better choice, with a higher slow and bonus health, however Rylai's Crystal Scepter may be taken if you expect the need of another AoE slow source and higher ability power.
There is no real reason to build a Frozen Mallet on Sejuani, as the slow will be largely outdone by her abilities, and there is more utility and tankiness to be gotten on other items.
Buying a Sunfire Cape allows for continous damage on enemies, used in combination with slows it can rack up a lot of damage over a period of time.
Much like Leona, building Sejuani as a full AP caster is not ideal due to her low AP ratios.
Building a Randuin's Omen on her is reccomended, as the Heart of Gold it needs is great for the extra gold and combining it with her other skills such as Permafrost and Arctic Assault can greatly slow and shut down the enemy team.
Randuin's Omen provides another extremely powerful active, as well as many of the stats Sejuani needs, and a passive anti-dps on-being-hit. This item is built from a Heart of Gold, another very useful early-mid game item.
Frozen Heart may be the most important item for Sejuani, as it provides a very large amount of armor and, more importantly, CDR, which helps deal with her very long cooldowns, on top of an extremely powerful defensive aura.
Guardian Angel is also a reasonable item for Sejuani late game given that her role in team fights is generally to charge straight into the enemy and hamper them with her AoE skills. Because of this, she's prone to death; Guardian Angel can be beneficial by reviving her occasionally.
Recommended items
Summoner's Rift recommended items
Starting
Essential
Offensive
Defensive
Consumables
Summoner's Rift recommended items
Starting
Essential
Offensive
Defensive
Consumables
Countering
Sejuani is an extremely potent ganker once she hits level 4. Refrain from boldly pushing your lane if you believe she's reached this level. Even before level 4, her ganks can still hurt, so always have a ward in place to prevent getting jumped.
Once she has a few levels going for her Sejuani is a very proficient farmer, so it would be wise to try and stop her from farming too much throughout the game, or she may become extremely difficult to deal with/kill later on.
Glacial Prison is a very powerful initiation ability, and if multiple teammates are caught in it, it could prove a difficult recovery. Avoid bunching up while she is near and/or making yourself an easy target.
Once Sejuani catches you alone, escape is nearly impossible. Avoid invading her jungle if she's made herself difficult to kill and you are unsure of her position. Also, avoid pushing lanes by yourself if you are not certain you can kill her or get away, or of her location.