I have 100% same experience with this fight. I recently posted this in Tales from Duty Finder thread:

Quote Originally Posted by Taika View Post
Healing Shinryu ex clear party as WHM. Well, trying to. People are dying to AoEs. I see I'm healing for 9k HPS, my partner AST for 5k. I can't say this so instead I say "I feel like I'm just spamming AoE heals and it's still not enough." AST says "Me too!" Arrrrrrgggggggggggggghhhhhhhhhhhh.
I admit from the start I'm not the best healer there is right now, since I'm currently BRD main, but I have mained it from Coil to Alexander and have healed OS1-OS3 and all previous Extremes, and know how to plan my ability use. I can handle my own part, but I'm not good enough to carry another healer through when they just. don't. heal. (or don't heal at correct times and refuse to apply shields). I hate saying at level 70 things like "you should use Medica II sometimes!" or "you should apply shield before the huge AoE hit!"... I've pretty much decided not to do this fight anymore without a healer partner I know and just farm OS4 weapons instead, that's probably easier anyways...