THIS WILL BE A HORSEY.

You're not alone, my BF and I both complain about healing this fight. TBH a lot of the difficulty comes from how competent the rest of your team is, because there's loads of damage to be avoided (and plenty that can't). If you can swing it, Adlo a DPS, use Deployment Tactics on them and get everybody that nice big shield before the stupid elf opens his dumb mouth and squeals about sickness needing to be purged. After that it's basically running from chains and horses.