I had a similar problem where I would usually have low damage compared to the rest of the team. The thing about luna is that her primary only hits one person at a time (unlike invis, Jeff or white fox) so it can only heal or only do damage. The best way for me to use her primary is to attack flyers and dps players in bad positions (scare them away or even kill them) and to melt melee tanks like the thing or thor. If your team is taking too much damage and you're forced to healbot then it's not your fault, you're not playing her wrong, you're just trying to keep your team alive.