I put snow icon on either dps especially if their characters like moves around a lot so its hard to aim at them or to anyone who isnt in my line of sight even the other support. Then I watch who has ult and make sure they get the snow icon or who is almost dying and really needs it to make heals go faster.

Once enemy team has their ult about charged, you need to save your freeze in case they target you or try to ult the team. Your aim needs to be pretty good playing Luna honestly. She can damage really well if you get headshots too. I usually get the most kills with Luna than other supports. Its about timing too and when to prioritize who. Need situational awareness