This really sounds like an issue with your healers. There's no reason for them to be aoe healing at all. I will if the ranged happen to wander too close to the raid and get us hit with flames but I opt for medica since it has more burst, the damage isn't consistent and know adds will be spawning.

Make sure your healers are sharing the pain in healing ranged and not just designating that to one person. I heal OT and one ranged while our scholar heals MT and one ranged (our group has two melee). We share any extra raid healing needed.