well you need to know why It is viable well its understandably easy sej has a kit that allows her to stay on her enemy without them getting away as well as an escape. She is great versus some of your normal hard guys like yorick or teemo even fiora. Irelia if highly skilled is a hassle but beatable same for Darius, nasus and singed. she does counter a lot of people top but there are some you need to worry about. Nidalee as an AD, AP nidalee is just spear dodging ad nid is highly annoying. Elise Elise's poke destroys most tanks so its a no brainer she kills sej. So care if you are new with her dont go straight balls deep versus a garen and other op tops. OH! Fair warning Dont try to keep up with CS you will get yourself killed instead of killing. Let the lane Come to you while gaining experience since Bristle is big its hard to maneuver around the minion waves which = death. so be patient and farm near tower. at around level 4-5 you can start considering balls deep killing.

P.S. I will show scores of my sejuani games using my build and also what happens if someone picks a counter to her how it can go. And these are games from silver 2-3

In my build I dont often get boots early like everyone else I actually rush a warmogs. start off with to reju's and 3 hp pots. the reju's will allow you good sustain for the start that way you can survive longer without destroying your pots. and as I said i usually hold back farm under tower and wait til 4-5 to kill. after you get enough for a giant's belt go back and buy it along with a couple of pots depending on how much you used before. and then go for more kills and by this time if you are doing well you should be able to deny them farm and get kills if they get greedy with your dive. and then get a warmogs and continue the build depending on what you are versing if its an ap top get a spirit visage first if its an ad top get a sunfire first either way you are getting those items. then the ancient golem item for cdr and more health as well as the tenacity and regens. now the last item randuins and the Mobi boots. Randuin's you can have for armor and health but you can also go with a thornmail as well if their team is ad heavy.

Well As runes go its quite simple she has very high armor already but at around level 8 with the runes she'll have almost 80 armor. all of her damage is Magic damage so the magic pen helps with that. and the cdr allows you to spam northern winds like theres no tomorrow.

Well with that full build if you are building entirely to the chart you should have.
5k health
200+ armor
100+ mr
high mobility
38% CDR
14 Mag. Pen
and very high health regen and a moderate mana regen


Where this calculates
The high cdr allows ability spamming which is a plus and i prefer a 63 second cd on ult rather than 110 seconds and her northern winds becomes constant.

so lets see:
Norther winds(W, main damage source) does at rank 5, 44(10% ap + 2% max hp) per second without frost. With frost it does 66(15% ap +3% max hp) per second. At the max build it does 66+150+15%ap per second thats 216+ damage not calculating magic pen or resistance of the enemy and with it constantly going its worse than mundos flames. as well as the base damage of the rest of her abilities I am easily able to flash past the full team dash damage the adc and focus them and then ulting the team as I make my escape after destroying the adc and the 5k health and high armor and mr makes it really hard to kill you.

18/6/16 game versus Xin.
4/4/11 Versus Vi
5/0 versus Shen
5/0 versus teemo
2/6 versus yorick(and a fed to **** vayne and mundo)
4/5 versus garen (lost by brand having an 18/2 game)

Same build everytime.