The last boss in Brayflox is a mixed bag as a healer. If the tank keeps getting the Dragon out of the poison in time, then it's a pretty quick fight. But if the tank does not, then the Dragon heals himself while standing in the poison. That will make it a long drawn out fight, and you don't have the mana to sustain healing such a fight.

I've failed the first time I've tried it. Trying to keep everyone up.

Now I'm basically only healing the tank, and DPS only when they blunder into a pool of poison. Use your Esuna whenever you can, especially on the tank. It cleanses the poison off the recipient. And that's damage you don't have to heal. Do use Esuna on a melee DPS like a Rogue, Lancer or a Pugilist though. They have to be close to the boss to deal damage, so they will take poison damage. And throw them a Cure if necessary. But your main priority is to keep the tank up.

On my Scholar mana is not a problem. Plenty of tricks to get mana back. But Conjurer/Whitemage doesn't have that luxury in Brayflox. Shroud of Saints is level 38, but is not usable in dungeons below that level because of level sync. So you will never have it in Brayflox, same as with Regen.

Oh and if all else fails, you can always use your limit break if it's available. That is if a DPS isn't using it.