The Serina Leader Add-on for Halo Wars 2 has 3 achievements worth 30 gamerscore.
Shatter 20 squads that have been Frozen. (Multiplayer, Skirmish or Blitz)
Win 3 matches as Serina (Multiplayer, Skirmish or Blitz)
Freeze 50 squads with Cryo Abilities (Multiplayer, Skirmish or Blitz)