It wasn't until after I've finished this encounter that I realised there was a sweet spot (at the back) where you can stand and avoid taking damage from both Unyielding Pain and Condemnation (Lady Blaumeux and Sir Zeliek's out of range AOE). Standing on the sweet spot could have relieved a lot of healing stress at the start of the fight.

Taking the sweet spot into consideration, the fight probably isn't that healing intensive, as long as one of the melee horsemen is nuked down as quickly as possible. Multi-dotting may seem to be an attractive option at first, but this is not recommended since that would be more healing required and less mana available to do so (since more mana is spent on multi-dotting).

Once the first horseman goes down, the rest of the fight is very manageable.