Sustain.

EXTREMELY useful. I use this at least once or twice a boss fight. Your fairy prioritizes herself last, so if people are taking constant damage, she probably won't ever get around to healing herself outside of using whispering dawn. It has such a short cast time, you just stop for a sec, cast, and continue moving. Almost like an instant cast, lol. I also use it if a tank derps and faces the boss/mobs toward the fairy and she gets smacked by an AoE. Boss attacks DO hurt the fairy quite a bit. Coil bosses can actually one shot her. (Stone-skinning your fairy is also a good idea for damage prevention before encounters). I've also had her pull aggro with a whispering dawn right off the bat in boss fights. Fun, lol.

Miasma II.

Use it during dungeons on mobs after you bane them, then Blizzard II spam.

On a single target mob, it's a DPS loss. Bio>Ruin II>Misama II on single target. Don't ever use Miasma II on single target.