I've played quite a bit of Seraphine, playing around with runes, builds, summoners, etc. So far, the best I've done is with this particular build. Rylai's removes the need for both Glacial and GLP, giving you room for more damage. It is actually a bit better overall too since it requires no setup.

If you hit E with Rylai it is an instant root every time. No shooting GLP, no basic attacking, no need for an ally with slows. It's all you, baby.

Plus Rylai throws in some extra health for survivability along with some big slows with Q when you need them.

This is just all my personal take on Sera, as none of the guides right now even mention Rylai posibilities.