Just an update: I just cleared this FP first time through with absolutely no deaths with my team at lvl 17/18!

Mmy team: Level 18 Assasin Tank, 2 Level 17 Snipers, Level 18 Sorc.

I was really surprised because I wasn't planning on even being able to clear it at that level. I followed my own advice from the above post by not moving away from the grenades and I beat the last boss pretty easily. Ranged DPS definitely makes this one easier. Also of note, I did focus down all the adds as they came to keep them off the healer.