I'm assuming that your healer is still at 5; at that augment level and at its 4th limit break, your stats are relatively low compared to a 6 unit at max limit break. At max stats, you should be able to tank a few hits so it's not too surprising that you're getting one shot at your level.
If you don't have the resources to augment and limit break your character further, I suggest you focus solely on healing so you don't draw any aggro and familiarize yourself with the bosses moveset so you can position yourself outside of its long range attacks. If you haven't already, please check out this guide for more info: