I echo miraidensestsu and think it was likely doom that got you if the issue was the last boss. The last boss has two main mechanics that are for the tank and healer. The first are the circle markers he places on the ground, which the tank needs to pull him out of. The second is doom. If you are already used to seeing debuffs with the blue bar above the icon that indicates the debuff can be cleansed, this version of doom does not have it. It cannot be cleansed, and the way that it is removed is by healing the inflicted party member to full. Pay attention to the status bars and you'll see a purple icon with a blue-ish black swirl on the inflicted player, which can be anyone in the party.

This particular dungeon and boss aren't necessarily a curve in difficulty for healing, but it will require for the healer to be paying more attention to status debuffs. For future reference, doom is a very common mechanic with voidsent type bosses. They can be some nasty buggers, so always keep an eye out for that icon when facing them.