If you want to stack them, having good dps is really a requirement. The add is 5-10% (Can't remember) health to the target it heals, so it will really increase the amount of damage you have to push out in order to get them down. For my main group we have them stacked up, then again we have people ranking top 50 consistently on WoL, so yeah.. In my alt group we keep the priest away and kill the add.