Ok, the first thing you need to be aware is the mechanics of those fights. As a healer, try to focus your healing on your party and the alliance Main tank. Ignore everyone else (later you'll be more experienced and you will solo heal an entire alliance if you want to, I'm a bad healer and I can do it)

For example. Heal pots in thanatos when they need it, keep your party's hp up in king behemoth and those basic stuff, any doubt just ask your party. If they are elitists, just ignore and try to learn by yourself