I've mentioned this before, but I don't mind doing so again...
For any quest in which we are required to protect someone, please allow us to add that person to the party. If they are in the party, we can monitor their red bar much more easily and target them for heals much more easily.
I spent part of this morning struggling through Hold for Reinforcements, solo (with a hireling targeted on defending Coyle) after after several fails I (once again) gave up and bought an overlevel hireling from the store so I could run it on Casual without breaking my streak.
Quests requiring the protection of another person should not be difficult because of wonky game mechanics. If you could add the protectee to the party (just like everyone else you might be healing at some point) that would be consistent with the story (Coyle becomes part of your group for two quests), so what's the problem?