I do not think you have read that ability of her Nothern Winds really carefully. Because it clearly states that the maximum health of HER (meaning Sejuani) is dealing the extra damage. That is why you see a ammount of extra damage at the tooltip. :) So stacking health is going to give her more damage with her W, not a Mardred.

it works pretty good on 5v5. the build makes it easy to make gold if you combine the items giving gold/10sec with some kills and assists, so you can build your items like the empire state building if you're a good player and get used to the hero a bit. you made a very good guide for a newcome champion. i'm sorry that the voting is not yet activated, but i'll thumb it up once available. thanks for the guide.

I like it a lot! Nice guide.