Hello, I've been playing a lot of AP/tank Sejuani recently and I think it is the best way to play her if you want to actually deal some damage and not be a stand alone tank CC bot. In the mid game with this build, you actually deal a fair amount of damage to everyone, with lots of CC and teamfighting capability. It is also a good pick for a stable and fairly gank pressure laning phase because you can either be the ganker or (if you are playing top) you have the CC available to set up the ganks or solo kills.

These items work the best for the Sejuani build. You could obviously switch some things out for something like a banshee's veil, but the example builds work the best in my opinion.
Runic Echoes: 80AP allows for better camp clear and the extra mana is useful if you hand over the second blue buff to your mid laner.
Nninja Tabi: Extra armour and movement speed. You don't need mercury's treads because of your passive
Rod of Ages: Lots of health, lots of AP, perfect for the build and it scales into the late game
Iceborn Gauntlet: Lots of armour and mana. The sheen proc is also very useful with your E and W for extra damage
Zhonya's Hourglass: The stopwatch utility is great for disrupting teamfights and avoiding damage if you ever get focused
Adaptive Helm: The magic resist and health regen keeps you tanky while your other items keep you dealing damage
Knight's Vow: The team utility is very useful and it also helps your own tanky stats

You want to be maxing your W, then your E, then your Q. You want to take specific abilities first however, like E for jungle sejuani, and Q second ability for top lane. The W max offers the most sustained damage and jungle clear/waveclear, then your E offers the aftershock proc and Stun with additional AP scaling damage.

Sejuani's runes are similar to that of a tank build for this build. Yet, with this build we want to be stronger than enemies as soon as possible, so we take Future's Market and Cosmic Insight from the Inspiration secondary tree. This benefits us because we can buy items earlier and have a better cooldown on our ultimate which is a good source of engage

Thank you for reading. I really enjoy playing AP Sejuani and I hope you do too. I have other guides for ADCs which I play and I have a blog if you want to read them