I think Serina needs a better third leader power spike, have shatter at 2, and freeze at 3. Move frost ravens to 3rd and heal to 4th.
Then you have two clear paths with complimentary power picks. 1st - Mines (or cubes), shatter, freeze, freeze 2 and Big Drop. 2nd - Mines (or Cubes) Cryogenics, frost ravans, turrent (with surrounding freeze), big drop/big freeze.
Also please make Frost Troopers better, I know they are great at harassing economy but the actually suck at killing buildings and other infantry.