There is one extremely easy way to counter these guys and that is, to not fight!
If you're a monk or have a monk secondary, whatever you just have to use a few moves and you're set.
1: Balthazar's Aura -- KEY - Mergoyles have pathetic attacks so really they won't do much dmg. Each time they attack you gain 1 energy.
2: Zealot's Fire -- KEY - With this you do not have to attack! Just sit there, take a beating and heal yourself over and over. You will regain energy, and everytime you heal all of the mergoyles surrounding you will take about 50 fire damage if you have points into smiting.
3: Symbol of Wrath -- If you have this, which you should if you're a monk, use this first.. only 5 energy and it does decent damage. Then let your energy regen.
EDIT: 4: Blathazar's Aura -- This is a droknar's forge level spell so most won't have it at this level but in any case, it also rocks

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The second option would be to just have a warrior run in and kill them, but if you're trying to solo what I've put in will help. I am personally a lvl 20 E/Mo and with this setup I can kill literally 20+ mergoyles all at the same time--and never run out of energy. No other profession can do that.
Anyway I just thought I'd share my info. As I was just out looting I thought hey why not share this info with others who may not be familiar with these moves.
GL and good looting.