Good advice. I'll add one of my own.
For anyone getting a heal, don't stand around like an idiot. Swing that weapon! A huge problem with the later levels is that enemies don't die nearly as quickly. People tend to rush in and take a ton of damage, then when they retreat, they bring the entire mob to...you guessed it, the healer. Healers can't heal inside a large mob, they'll get knocked around and spells will be canceled. At 300+ mp for a Light of Healing, it's not something we can afford. Keep that in mind.