The adds only have a limited time agro table then it resets you cant really tank them.
Tank him right at the base of the forge so the healer can stand along way back make sure you hold agro on him cause the dps will be burning like crazy and when adds look like they are getting close to healer use RD on healer and they will start walking back towards you.
Even if the healer got killed you should be able to survive unhealed in your current gear he doesnt hit that hard.
There are only usually 2 reasons for wipe :
1.) You have a low dps group as he gets low he decices to explode the adds they get frozen for a few seconds and then explode. If healer and or dps don't move away from them when he does this they will die its like the ghouls in TOC 5 man. High dps groups will kill him before he can explode the statues.
2.) You lose agro on him and he kills the dps and or healer. This is your main priority keep agro on him so the dps can unload.
So which 1 of the 2 ways occurred because you are kind of misinterreptting what went wrong?
** Remember Warcraft players fail in directions you never thought possible.