You can reduce Sejuani’s ability to get successful ganks of by warding her jungle entrances or placing wards in the river. If you know where she is, she will be unable to get good ganks off.
Sejuani has good Drake control in the early game. Ensure that you take the bottom side scuttle crab and keep the Drake warded at all times so she is unable to sneak it away from your team.
Once Sejuani hits level 6 and unlocks her Ultimate R, she will be looking to gank as often as she can. You could delay this power spike by invading her in the early game or stealing away her camps when she is spotted on the other side of the map.
Sejuani is very good at picking people off with her Ultimate R at the start of the fight. Make sure no one is too far forward especially the carries to make it harder for her to get a pick.
Avoid splitting up when playing against a Sejuani. If you are not grouping, she will just force a fight with her Ultimate R, so make sure you’re grouped as a team.
Poke, disengage and CC will reduce Sejuani’s ability to fight. If possible, try to harass and poke her down from afar to make it incredibly difficult to use her Q and engage.
Once Sejuani hits level 6, her kill pressure increases dramatically as her Ultimate R is a great ganking tool. Expect her to gank whenever her Ultimate R is up.
As soon as Sejuani has completed her jungling item, it will be difficult for you to kill her as she will be rather tanky. Avoid fighting her at this time unless you have an advantage.
Sejuani is really good during the mid and late game as she can catch players out in a team fight with her Ultimate R. Watch your positioning so she cannot isolate you and start the team fight.