Sounds like your healer just wasn't used to it, or the people weren't equipped properly equipped (enought defence stat)
It can be a hard fight for a healer of the people take big damage. Medica is their friend.
I'd say you've been unlucky with your team, by the sounds of it.

Also DPS needs to burn him down ASAP.

You know the mechanism by now, so I'd say try a shout out for help from people who have done it before, or an FC. Or try DF again, and hope for a better healer (dare I say it)

I would offer, but we're not on the same server