Ok, here is my Moloch strategy, which also works for characters under 20. First off, you need to have an area of effect damage skill. This is a must, and I will explain why:
When Moloch summons his healers, they immediately start healing his damage. That's all they do as long as he is not at full health. If he is at full health, they heal each other.
So, the idea is to keep damaging Moloch while you pick off the healers one by one. If you have a class that does not have a damage over time skill, like a guardian, you have to do enough damage to Moloch that it gives you a chance to start banging away on a priest. Keep an eye on Moloch's health, because you never want to let it return to full strength unless there is only one priest left to kill.
The healers are usually summoned roughly twice, but not all of them appear at the same time, there might be a minute or two before all of them show up.
Using an area of effect damage skill is a bonus because not only are you hurting Moloch, but doing damage to the priests as well.
Very low level characters will still have a hard time, because of Moloch's damage output, but so far, I have been able to beat him with guardian, evoker and marksman right around the 17-18 level range using this strategy.
tldr: Always keep Molly at less than full health. Beat on a priest until Molly is almost all healed, then switch back to damage Molly some more, go back to beating on priest till it's dead. Rinse and repeat.
