Hello,
my name is Joe and this is my first guide, I am currently unranked but before the new season came in I was Silver III. I have been playing alot of sejuani so I hope we can share opinions and playstyles to keep sejuani from becoming a champion that doesnt see much play.

Alot of people build Sejuani full tank while building this way it will be hard to kill Sejuani. But building only HP and resistances you are only using the HP scaling from the second cast of your W and are missing five AP scalings on your Abilities (Did not include targets %HP Dmg on first cast of W). Don't get me wrong the utility plus the tank stats are nice but you are only using a sixth of her kits scaling, so in my opinion it's a waste to not build AP.

Scaling:
Q - 40% AP (Decent scaling but this is more utility)
W - First cast, 4 / 4.5 / 5 / 5.5 / 6% of targets max HP (+3% per 100 AP)
Second cast, (Total Dmg over 4 seconds) + 4 / 6 / 8 / 10 / 12% maximum health (+60% AP)
E - 50% AP
R - 80% AP

Thank you for taking a look at this guide I hope I helped you with your build. Any suggestion are welcome I will make sure to credit anybody in the guide with their suggestions.