I know that is the third day and might be soon to call her OP, but there are two things that I think should be adressed already.
1st The Cryotech Advances passive is way too early in the game for all the advantages it offers, and I say that because we do have other passives with similar buffs: Anders’ Sentinel Beacon and Isabel’s best defense, both of these passives are on Tier 4, but Serina has a merge of the two on Tier 2 and also adds some of Atriox’s countermeasures, I think that this either requires that the power is pushed to T4 or being split on multiple levels.
The second thing is that Frostravens and Cryotroopers are seemingly able to shoot throug base shields, the Kodiak’s and Bison’s chill effect is able to ignore it as well. While even Inferno had to take down the shields first in order to affect the base.
Out of that, the only problem I have with her is the frustration of see that units are frost every fourth step, but I think that’s how she’s meant to act and I still have to find the counter for that.