Problem with healer is when 2 or more mobs starting to attack you. One of them for sure will start attacking the healer when the healer use heal or assist. So here you need to shield the healer and try to switch attacker from the healer to your main, while other mobs still pounding you. Say you make all enemies focus on you, but then if you need another heal, and some mobs will again attack your healer and so on.