About Sejuani

She's still quite strong in my experience and able to carry games even in Platinum.

She may not kill people as quickly as she used to, but she really shouldn't considering how much CC and durability she has.

She's still strong at initiating teamfights and providing a consistent damage threat.

How are you building her and playing her?

I like to use 0/12/18 Masteries (with Strength of Ages keystone)

Runes are MS Quints/AtkSpd Marks/Flat Armor Seals/Scaling MR Glyphs.

I tend to prioritize CDR, so my Item build will often look something like the following:

If you want, you could probably get away with health stacking though:

Though if you take the health-stacking approach, I would probably take Scaling CDR Glyphs.

She doesn't need to.