Many bosses and some of the larger enemies (dullahans) are immune to the knockback of Fluid Aura. I think it's okay to use it on them in such cases, although you have to get in melee range. Some bosses such as AK and ST bring you close enough to them to use it as part of the game mechanic. Other bosses like those in coil are just too dangerous or heal-intensive for the healer to get that close.